From 3bc775fcfe53609c384c877665be179560c8b1be Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Sun, 14 Aug 2016 18:15:47 -0700
Subject: [PATCH 1/7] Prefer Either over Xor, fixes #1192

This commit keeps Xor and XorT but moves most Cats functions from Xor to Either.
Xor will be removed after 0.8.0.

- Add methods from Xor onto enrichment class of Either
- Copy XorT into an EitherT based around Either
- Styling: Occurences of Either are styled as Either[A, B] instead of A Either B (as it was in Xor)
---
 .../main/scala/cats/ApplicativeError.scala    |  16 +-
 core/src/main/scala/cats/Eval.scala           |   7 +-
 core/src/main/scala/cats/FlatMapRec.scala     |   6 +-
 core/src/main/scala/cats/Foldable.scala       |  27 +-
 core/src/main/scala/cats/MonadCombine.scala   |   2 +-
 core/src/main/scala/cats/Traverse.scala       |  15 +-
 core/src/main/scala/cats/arrow/Choice.scala   |  20 +-
 .../src/main/scala/cats/arrow/FunctionK.scala |   6 +-
 core/src/main/scala/cats/data/Coproduct.scala |  17 +-
 core/src/main/scala/cats/data/EitherT.scala   | 459 ++++++++++++++++++
 core/src/main/scala/cats/data/Ior.scala       |  14 +-
 core/src/main/scala/cats/data/Kleisli.scala   |   2 +-
 .../main/scala/cats/data/NonEmptyList.scala   |   8 +-
 .../main/scala/cats/data/NonEmptyVector.scala |   8 +-
 core/src/main/scala/cats/data/OptionT.scala   |  15 +-
 core/src/main/scala/cats/data/StateT.scala    |   4 +-
 core/src/main/scala/cats/data/Validated.scala |  10 +-
 core/src/main/scala/cats/data/Xor.scala       |   8 +-
 core/src/main/scala/cats/data/XorT.scala      |  12 +-
 .../main/scala/cats/instances/either.scala    |  29 +-
 .../main/scala/cats/instances/function.scala  |   7 +-
 .../main/scala/cats/instances/future.scala    |  12 +-
 core/src/main/scala/cats/instances/list.scala |  10 +-
 .../main/scala/cats/instances/option.scala    |   7 +-
 core/src/main/scala/cats/instances/try.scala  |  11 +-
 core/src/main/scala/cats/package.scala        |   7 +-
 .../scala/cats/syntax/applicativeError.scala  |   6 +-
 core/src/main/scala/cats/syntax/either.scala  | 208 +++++++-
 core/src/main/scala/cats/syntax/flatMap.scala |   5 +-
 .../main/scala/cats/syntax/monadCombine.scala |   3 +-
 docs/src/main/tut/validated.md                |  12 +-
 free/src/main/scala/cats/free/Free.scala      |  23 +-
 free/src/test/scala/cats/free/FreeTests.scala |   2 +-
 .../test/scala/cats/free/InjectTests.scala    |   6 +-
 .../cats/laws/ApplicativeErrorLaws.scala      |  14 +-
 .../src/main/scala/cats/laws/ChoiceLaws.scala |   3 +-
 .../main/scala/cats/laws/FlatMapRecLaws.scala |   5 +-
 .../discipline/ApplicativeErrorTests.scala    |   8 +-
 .../cats/laws/discipline/ChoiceTests.scala    |   3 +-
 .../laws/discipline/MonadErrorTests.scala     |   8 +-
 .../cats/tests/ApplicativeErrorTests.scala    |  14 +-
 .../scala/cats/tests/CoproductTests.scala     |   4 +-
 .../src/test/scala/cats/tests/IorTests.scala  |   8 +-
 .../test/scala/cats/tests/KleisliTests.scala  |   6 +-
 .../test/scala/cats/tests/ListWrapper.scala   |   2 +-
 .../cats/tests/MonadRecInstancesTests.scala   |   8 +-
 .../test/scala/cats/tests/OptionTTests.scala  |  48 +-
 .../scala/cats/tests/ValidatedTests.scala     |  10 +-
 .../test/scala/cats/tests/WriterTTests.scala  |   8 +-
 .../src/test/scala/cats/tests/XorTTests.scala |   6 +-
 .../src/test/scala/cats/tests/XorTests.scala  |   4 +-
 51 files changed, 911 insertions(+), 252 deletions(-)
 create mode 100644 core/src/main/scala/cats/data/EitherT.scala

diff --git a/core/src/main/scala/cats/ApplicativeError.scala b/core/src/main/scala/cats/ApplicativeError.scala
index f575c7f703..c6c751b696 100644
--- a/core/src/main/scala/cats/ApplicativeError.scala
+++ b/core/src/main/scala/cats/ApplicativeError.scala
@@ -1,6 +1,6 @@
 package cats
 
-import cats.data.{Xor, XorT}
+import cats.data.EitherT
 
 /**
  * An applicative that also allows you to raise and or handle an error value.
@@ -35,21 +35,21 @@ trait ApplicativeError[F[_], E] extends Applicative[F] {
   def handleError[A](fa: F[A])(f: E => A): F[A] = handleErrorWith(fa)(f andThen pure)
 
   /**
-   * Handle errors by turning them into [[cats.data.Xor.Left]] values.
+   * Handle errors by turning them into [[scala.util.Either]] values.
    *
-   * If there is no error, then an [[cats.data.Xor.Right]] value will be returned instead.
+   * If there is no error, then an [[scala.util.Right]] value will be returned instead.
    *
    * All non-fatal errors should be handled by this method.
    */
-  def attempt[A](fa: F[A]): F[E Xor A] = handleErrorWith(
-    map(fa)(Xor.right[E, A])
-  )(e => pure(Xor.left(e)))
+  def attempt[A](fa: F[A]): F[Either[E, A]] = handleErrorWith(
+    map(fa)(Right(_): Either[E, A])
+  )(e => pure(Left(e)))
 
   /**
-   * Similar to [[attempt]], but wraps the result in a [[cats.data.XorT]] for
+   * Similar to [[attempt]], but wraps the result in a [[cats.data.EitherT]] for
    * convenience.
    */
-  def attemptT[A](fa: F[A]): XorT[F, E, A] = XorT(attempt(fa))
+  def attemptT[A](fa: F[A]): EitherT[F, E, A] = EitherT(attempt(fa))
 
   /**
    * Recover from certain errors by mapping them to an `A` value.
diff --git a/core/src/main/scala/cats/Eval.scala b/core/src/main/scala/cats/Eval.scala
index 82a3a1a267..1687ff83e8 100644
--- a/core/src/main/scala/cats/Eval.scala
+++ b/core/src/main/scala/cats/Eval.scala
@@ -1,7 +1,6 @@
 package cats
 
 import scala.annotation.tailrec
-import cats.data.Xor
 import cats.syntax.all._
 
 /**
@@ -302,10 +301,10 @@ private[cats] trait EvalInstances extends EvalInstances0 {
       def flatMap[A, B](fa: Eval[A])(f: A => Eval[B]): Eval[B] = fa.flatMap(f)
       def extract[A](la: Eval[A]): A = la.value
       def coflatMap[A, B](fa: Eval[A])(f: Eval[A] => B): Eval[B] = Later(f(fa))
-      def tailRecM[A, B](a: A)(f: A => Eval[A Xor B]): Eval[B] =
+      def tailRecM[A, B](a: A)(f: A => Eval[Either[A, B]]): Eval[B] =
         f(a).flatMap(_ match {
-          case Xor.Left(a1) => tailRecM(a1)(f) // recursion OK here, since flatMap is lazy
-          case Xor.Right(b) => Eval.now(b)
+          case Left(a1) => tailRecM(a1)(f) // recursion OK here, since flatMap is lazy
+          case Right(b) => Eval.now(b)
         })
     }
 
diff --git a/core/src/main/scala/cats/FlatMapRec.scala b/core/src/main/scala/cats/FlatMapRec.scala
index 5658b3fe00..5bc95a8beb 100644
--- a/core/src/main/scala/cats/FlatMapRec.scala
+++ b/core/src/main/scala/cats/FlatMapRec.scala
@@ -2,8 +2,6 @@ package cats
 
 import simulacrum.typeclass
 
-import cats.data.Xor
-
 /**
  * Version of [[cats.FlatMap]] capable of stack-safe recursive `flatMap`s.
  *
@@ -13,12 +11,12 @@ import cats.data.Xor
 @typeclass trait FlatMapRec[F[_]] extends FlatMap[F] {
 
   /**
-   * Keeps calling `f` until a `[[cats.data.Xor.Right Right]][B]` is returned.
+   * Keeps calling `f` until a `[[scala.util.Right]][B]` is returned.
    *
    * Implementations of this method must use constant stack space.
    *
    * `f` must use constant stack space. (It is OK to use a constant number of
    * `map`s and `flatMap`s inside `f`.)
    */
-  def tailRecM[A, B](a: A)(f: A => F[A Xor B]): F[B]
+  def tailRecM[A, B](a: A)(f: A => F[Either[A, B]]): F[B]
 }
diff --git a/core/src/main/scala/cats/Foldable.scala b/core/src/main/scala/cats/Foldable.scala
index 00c63a38da..acb89624dd 100644
--- a/core/src/main/scala/cats/Foldable.scala
+++ b/core/src/main/scala/cats/Foldable.scala
@@ -186,9 +186,8 @@ import simulacrum.typeclass
    * For example:
    *
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> def parseInt(s: String): Option[Int] = Xor.catchOnly[NumberFormatException](s.toInt).toOption
+   * scala> def parseInt(s: String): Option[Int] = Either.catchOnly[NumberFormatException](s.toInt).toOption
    * scala> val F = Foldable[List]
    * scala> F.traverse_(List("333", "444"))(parseInt)
    * res0: Option[Unit] = Some(())
@@ -208,19 +207,18 @@ import simulacrum.typeclass
   /**
    * Behaves like traverse_, but uses [[Unapply]] to find the
    * [[Applicative]] instance for `G` - used when `G` is a
-   * type constructor with two or more parameters such as [[cats.data.Xor]]
+   * type constructor with two or more parameters such as [[scala.util.Either]]
    *
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> def parseInt(s: String): Xor[String, Int] =
-   *      |   try { Xor.Right(s.toInt) }
-   *      |   catch { case _: NumberFormatException => Xor.Left("boo") }
+   * scala> def parseInt(s: String): Either[String, Int] =
+   *      |   try { Right(s.toInt) }
+   *      |   catch { case _: NumberFormatException => Left("boo") }
    * scala> val F = Foldable[List]
    * scala> F.traverseU_(List("333", "444"))(parseInt)
-   * res0: Xor[String, Unit] = Right(())
+   * res0: Either[String, Unit] = Right(())
    * scala> F.traverseU_(List("333", "zzz"))(parseInt)
-   * res1: Xor[String, Unit] = Left(boo)
+   * res1: Either[String, Unit] = Left(boo)
    * }}}
    *
    * Note that using `traverse_` instead of `traverseU_` would not compile without
@@ -253,16 +251,15 @@ import simulacrum.typeclass
   /**
    * Behaves like sequence_, but uses [[Unapply]] to find the
    * [[Applicative]] instance for `G` - used when `G` is a
-   * type constructor with two or more parameters such as [[cats.data.Xor]]
+   * type constructor with two or more parameters such as [[scala.util.Either]]
    *
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
    * scala> val F = Foldable[List]
-   * scala> F.sequenceU_(List(Xor.right[String, Int](333), Xor.Right(444)))
-   * res0: Xor[String, Unit] = Right(())
-   * scala> F.sequenceU_(List(Xor.right[String, Int](333), Xor.Left("boo")))
-   * res1: Xor[String, Unit] = Left(boo)
+   * scala> F.sequenceU_(List(Either.right[String, Int](333), Right(444)))
+   * res0: Either[String, Unit] = Right(())
+   * scala> F.sequenceU_(List(Either.right[String, Int](333), Left("boo")))
+   * res1: Either[String, Unit] = Left(boo)
    * }}}
    *
    * Note that using `sequence_` instead of `sequenceU_` would not compile without
diff --git a/core/src/main/scala/cats/MonadCombine.scala b/core/src/main/scala/cats/MonadCombine.scala
index 55cb76184e..a418cba774 100644
--- a/core/src/main/scala/cats/MonadCombine.scala
+++ b/core/src/main/scala/cats/MonadCombine.scala
@@ -11,7 +11,7 @@ import simulacrum.typeclass
    * Fold over the inner structure to combine all of the values with
    * our combine method inherited from MonoidK. The result is for us
    * to accumulate all of the "interesting" values of the inner G, so
-   * if G is Option, we collect all the Some values, if G is Xor,
+   * if G is Option, we collect all the Some values, if G is Either,
    * we collect all the Right values, etc.
    */
   def unite[G[_], A](fga: F[G[A]])(implicit G: Foldable[G]): F[A] =
diff --git a/core/src/main/scala/cats/Traverse.scala b/core/src/main/scala/cats/Traverse.scala
index 9aef6df102..7e34eef82b 100644
--- a/core/src/main/scala/cats/Traverse.scala
+++ b/core/src/main/scala/cats/Traverse.scala
@@ -22,9 +22,8 @@ import simulacrum.typeclass
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> def parseInt(s: String): Option[Int] = Xor.catchOnly[NumberFormatException](s.toInt).toOption
+   * scala> def parseInt(s: String): Option[Int] = Either.catchOnly[NumberFormatException](s.toInt).toOption
    * scala> List("1", "2", "3").traverse(parseInt)
    * res0: Option[List[Int]] = Some(List(1, 2, 3))
    * scala> List("1", "two", "3").traverse(parseInt)
@@ -39,14 +38,13 @@ import simulacrum.typeclass
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> def parseInt(s: String): Xor[String, Int] = Xor.catchOnly[NumberFormatException](s.toInt).leftMap(_ => "no number")
+   * scala> def parseInt(s: String): Either[String, Int] = Either.catchOnly[NumberFormatException](s.toInt).leftMap(_ => "no number")
    * scala> val ns = List("1", "2", "3")
    * scala> ns.traverseU(parseInt)
-   * res0: Xor[String, List[Int]] = Right(List(1, 2, 3))
-   * scala> ns.traverse[Xor[String, ?], Int](parseInt)
-   * res1: Xor[String, List[Int]] = Right(List(1, 2, 3))
+   * res0: Either[String, List[Int]] = Right(List(1, 2, 3))
+   * scala> ns.traverse[Either[String, ?], Int](parseInt)
+   * res1: Either[String, List[Int]] = Right(List(1, 2, 3))
    * }}}
    */
   def traverseU[A, GB](fa: F[A])(f: A => GB)(implicit U: Unapply[Applicative, GB]): U.M[F[U.A]] =
@@ -57,9 +55,8 @@ import simulacrum.typeclass
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> def parseInt(s: String): Option[Int] = Xor.catchOnly[NumberFormatException](s.toInt).toOption
+   * scala> def parseInt(s: String): Option[Int] = Either.catchOnly[NumberFormatException](s.toInt).toOption
    * scala> val x = Option(List("1", "two", "3"))
    * scala> x.traverseM(_.map(parseInt))
    * res0: List[Option[Int]] = List(Some(1), None, Some(3))
diff --git a/core/src/main/scala/cats/arrow/Choice.scala b/core/src/main/scala/cats/arrow/Choice.scala
index df529e00cc..5954f1ede0 100644
--- a/core/src/main/scala/cats/arrow/Choice.scala
+++ b/core/src/main/scala/cats/arrow/Choice.scala
@@ -1,8 +1,6 @@
 package cats
 package arrow
 
-import cats.data.Xor
-
 trait Choice[F[_, _]] extends Category[F] {
 
   /**
@@ -12,20 +10,19 @@ trait Choice[F[_, _]] extends Category[F] {
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
    * scala> val b: Boolean => String = _ + " is a boolean"
    * scala> val i: Int => String =  _ + " is an integer"
-   * scala> val f: (Boolean Xor Int) => String = Choice[Function1].choice(b, i)
+   * scala> val f: (Either[Boolean, Int]) => String = Choice[Function1].choice(b, i)
    *
-   * scala> f(Xor.right(3))
+   * scala> f(Right(3))
    * res0: String = 3 is an integer
    *
-   * scala> f(Xor.left(false))
+   * scala> f(Left(false))
    * res0: String = false is a boolean
    * }}}
    */
-  def choice[A, B, C](f: F[A, C], g: F[B, C]): F[Xor[A, B], C]
+  def choice[A, B, C](f: F[A, C], g: F[B, C]): F[Either[A, B], C]
 
   /**
    * An `F` that, given a source `A` on either the right or left side, will
@@ -33,18 +30,17 @@ trait Choice[F[_, _]] extends Category[F] {
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> val f: (Int Xor Int) => Int = Choice[Function1].codiagonal[Int]
+   * scala> val f: (Either[Int, Int]) => Int = Choice[Function1].codiagonal[Int]
    *
-   * scala> f(Xor.right(3))
+   * scala> f(Right(3))
    * res0: Int = 3
    *
-   * scala> f(Xor.left(3))
+   * scala> f(Left(3))
    * res1: Int = 3
    * }}}
    */
-  def codiagonal[A]: F[Xor[A, A], A] = choice(id, id)
+  def codiagonal[A]: F[Either[A, A], A] = choice(id, id)
 }
 
 object Choice {
diff --git a/core/src/main/scala/cats/arrow/FunctionK.scala b/core/src/main/scala/cats/arrow/FunctionK.scala
index eaadb4d598..4cf392d3ff 100644
--- a/core/src/main/scala/cats/arrow/FunctionK.scala
+++ b/core/src/main/scala/cats/arrow/FunctionK.scala
@@ -1,7 +1,7 @@
 package cats
 package arrow
 
-import cats.data.{Xor, Coproduct}
+import cats.data. Coproduct
 
 trait FunctionK[F[_], G[_]] extends Serializable { self =>
   def apply[A](fa: F[A]): G[A]
@@ -17,8 +17,8 @@ trait FunctionK[F[_], G[_]] extends Serializable { self =>
   def or[H[_]](h: FunctionK[H, G]): FunctionK[Coproduct[F, H, ?], G] =
     new FunctionK[Coproduct[F, H, ?], G] {
       def apply[A](fa: Coproduct[F, H, A]): G[A] = fa.run match {
-        case Xor.Left(ff) => self(ff)
-        case Xor.Right(gg) => h(gg)
+        case Left(ff) => self(ff)
+        case Right(gg) => h(gg)
       }
     }
 }
diff --git a/core/src/main/scala/cats/data/Coproduct.scala b/core/src/main/scala/cats/data/Coproduct.scala
index 35b4f8a638..cc552b6c0d 100644
--- a/core/src/main/scala/cats/data/Coproduct.scala
+++ b/core/src/main/scala/cats/data/Coproduct.scala
@@ -3,12 +3,13 @@ package data
 
 import cats.arrow.FunctionK
 import cats.functor.Contravariant
+import cats.syntax.either._
 
-/** `F` on the left and `G` on the right of [[Xor]].
+/** `F` on the left and `G` on the right of [[scala.util.Either]].
  *
- * @param run The underlying [[Xor]].
+ * @param run The underlying [[scala.util.Either]].
  */
-final case class Coproduct[F[_], G[_], A](run: F[A] Xor G[A]) {
+final case class Coproduct[F[_], G[_], A](run: Either[F[A], G[A]]) {
 
   import Coproduct._
 
@@ -86,17 +87,17 @@ final case class Coproduct[F[_], G[_], A](run: F[A] Xor G[A]) {
 object Coproduct extends CoproductInstances {
 
   def leftc[F[_], G[_], A](x: F[A]): Coproduct[F, G, A] =
-    Coproduct(Xor.left(x))
+    Coproduct(Left(x))
 
   def rightc[F[_], G[_], A](x: G[A]): Coproduct[F, G, A] =
-    Coproduct(Xor.right(x))
+    Coproduct(Right(x))
 
   final class CoproductLeft[G[_]] private[Coproduct] {
-    def apply[F[_], A](fa: F[A]): Coproduct[F, G, A] = Coproduct(Xor.left(fa))
+    def apply[F[_], A](fa: F[A]): Coproduct[F, G, A] = Coproduct(Left(fa))
   }
 
   final class CoproductRight[F[_]] private[Coproduct] {
-    def apply[G[_], A](ga: G[A]): Coproduct[F, G, A] = Coproduct(Xor.right(ga))
+    def apply[G[_], A](ga: G[A]): Coproduct[F, G, A] = Coproduct(Right(ga))
   }
 
   def left[G[_]]: CoproductLeft[G] = new CoproductLeft[G]
@@ -106,7 +107,7 @@ object Coproduct extends CoproductInstances {
 
 private[data] sealed abstract class CoproductInstances3 {
 
-  implicit def catsDataEqForCoproduct[F[_], G[_], A](implicit E: Eq[F[A] Xor G[A]]): Eq[Coproduct[F, G, A]] =
+  implicit def catsDataEqForCoproduct[F[_], G[_], A](implicit E: Eq[Either[F[A], G[A]]]): Eq[Coproduct[F, G, A]] =
     Eq.by(_.run)
 
   implicit def catsDataFunctorForCoproduct[F[_], G[_]](implicit F0: Functor[F], G0: Functor[G]): Functor[Coproduct[F, G, ?]] =
diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
new file mode 100644
index 0000000000..d7329b1fa7
--- /dev/null
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -0,0 +1,459 @@
+package cats
+package data
+
+import cats.functor.Bifunctor
+import cats.instances.either._
+import cats.syntax.either._
+
+/**
+ * Transformer for `Either`, allowing the effect of an arbitrary type constructor `F` to be combined with the
+ * fail-fast effect of `Either`.
+ *
+ * `EitherT[F, A, B]` wraps a value of type `F[Either[A, B]]`. An `F[C]` can be lifted in to `EitherT[F, A, C]` via `EitherT.right`,
+ * and lifted in to a `EitherT[F, C, B]` via `EitherT.left`.
+ */
+final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
+
+  def fold[C](fa: A => C, fb: B => C)(implicit F: Functor[F]): F[C] = F.map(value)(_.fold(fa, fb))
+
+  def isLeft(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.isLeft)
+
+  def isRight(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.isRight)
+
+  def swap(implicit F: Functor[F]): EitherT[F, B, A] = EitherT(F.map(value)(_.swap))
+
+  def getOrElse[BB >: B](default: => BB)(implicit F: Functor[F]): F[BB] = F.map(value)(_.getOrElse(default))
+
+  def getOrElseF[BB >: B](default: => F[BB])(implicit F: Monad[F]): F[BB] = {
+    F.flatMap(value) {
+      case Left(_) => default
+      case Right(b) => F.pure(b)
+    }
+  }
+
+  def orElse[AA, BB >: B](default: => EitherT[F, AA, BB])(implicit F: Monad[F]): EitherT[F, AA, BB] = {
+    EitherT(F.flatMap(value) {
+      case Left(_) => default.value
+      case r @ Right(_) => F.pure(r.asInstanceOf[Either[AA, BB]])
+    })
+  }
+
+  def recover(pf: PartialFunction[A, B])(implicit F: Functor[F]): EitherT[F, A, B] =
+    EitherT(F.map(value)(_.recover(pf)))
+
+  def recoverWith(pf: PartialFunction[A, EitherT[F, A, B]])(implicit F: Monad[F]): EitherT[F, A, B] =
+    EitherT(F.flatMap(value) {
+      case Left(a) if pf.isDefinedAt(a) => pf(a).value
+      case other => F.pure(other)
+    })
+
+  def valueOr[BB >: B](f: A => BB)(implicit F: Functor[F]): F[BB] = fold(f, identity)
+
+  def forall(f: B => Boolean)(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.forall(f))
+
+  def exists(f: B => Boolean)(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.exists(f))
+
+  def ensure[AA >: A](onFailure: => AA)(f: B => Boolean)(implicit F: Functor[F]): EitherT[F, AA, B] = EitherT(F.map(value)(_.ensure(onFailure)(f)))
+
+  def toOption(implicit F: Functor[F]): OptionT[F, B] = OptionT(F.map(value)(_.toOption))
+
+  def to[G[_]](implicit F: Functor[F], G: Alternative[G]): F[G[B]] =
+    F.map(value)(_.to[G, B])
+
+  def collectRight(implicit F: MonadCombine[F]): F[B] =
+    F.flatMap(value)(_.to[F, B])
+
+  def bimap[C, D](fa: A => C, fb: B => D)(implicit F: Functor[F]): EitherT[F, C, D] = EitherT(F.map(value)(_.bimap(fa, fb)))
+
+  def bitraverse[G[_], C, D](f: A => G[C], g: B => G[D])(implicit traverseF: Traverse[F], applicativeG: Applicative[G]): G[EitherT[F, C, D]] =
+    applicativeG.map(traverseF.traverse(value)(axb => Bitraverse[Either].bitraverse(axb)(f, g)))(EitherT.apply)
+
+  def applyAlt[D](ff: EitherT[F, A, B => D])(implicit F: Apply[F]): EitherT[F, A, D] =
+    EitherT[F, A, D](F.map2(this.value, ff.value)((xb, xbd) => Apply[Either[A, ?]].ap(xbd)(xb)))
+
+  def flatMap[AA >: A, D](f: B => EitherT[F, AA, D])(implicit F: Monad[F]): EitherT[F, AA, D] =
+    EitherT(F.flatMap(value) {
+      case l @ Left(_) => F.pure(l.asInstanceOf[Either[AA, D]])
+      case Right(b) => f(b).value
+    })
+
+  def flatMapF[AA >: A, D](f: B => F[Either[AA, D]])(implicit F: Monad[F]): EitherT[F, AA, D] =
+    flatMap(f andThen EitherT.apply)
+
+  def transform[C, D](f: Either[A, B] => Either[C, D])(implicit F: Functor[F]): EitherT[F, C, D] =
+    EitherT(F.map(value)(f))
+
+  def subflatMap[AA >: A, D](f: B => Either[AA, D])(implicit F: Functor[F]): EitherT[F, AA, D] =
+    transform(_.flatMap(f))
+
+  def map[D](f: B => D)(implicit F: Functor[F]): EitherT[F, A, D] = bimap(identity, f)
+
+  def semiflatMap[D](f: B => F[D])(implicit F: Monad[F]): EitherT[F, A, D] =
+    flatMap(b => EitherT.right[F, A, D](f(b)))
+
+  def leftMap[C](f: A => C)(implicit F: Functor[F]): EitherT[F, C, B] = bimap(f, identity)
+
+  def compare(that: EitherT[F, A, B])(implicit o: Order[F[Either[A, B]]]): Int =
+    o.compare(value, that.value)
+
+  def partialCompare(that: EitherT[F, A, B])(implicit p: PartialOrder[F[Either[A, B]]]): Double =
+    p.partialCompare(value, that.value)
+
+  def ===(that: EitherT[F, A, B])(implicit eq: Eq[F[Either[A, B]]]): Boolean =
+    eq.eqv(value, that.value)
+
+  def traverse[G[_], D](f: B => G[D])(implicit traverseF: Traverse[F], applicativeG: Applicative[G]): G[EitherT[F, A, D]] =
+    applicativeG.map(traverseF.traverse(value)(axb => Traverse[Either[A, ?]].traverse(axb)(f)))(EitherT.apply)
+
+  def foldLeft[C](c: C)(f: (C, B) => C)(implicit F: Foldable[F]): C =
+    F.foldLeft(value, c)((c, axb) => axb.foldLeft(c)(f))
+
+  def foldRight[C](lc: Eval[C])(f: (B, Eval[C]) => Eval[C])(implicit F: Foldable[F]): Eval[C] =
+    F.foldRight(value, lc)((axb, lc) => axb.foldRight(lc)(f))
+
+  def merge[AA >: A](implicit ev: B <:< AA, F: Functor[F]): F[AA] = F.map(value)(_.fold(identity, ev.apply))
+
+  /**
+   * Similar to [[Either.combine]] but mapped over an `F` context.
+   *
+   * Examples:
+   * {{{
+   * scala> import cats.data.EitherT
+   * scala> import cats.implicits._
+   * scala> val l1: EitherT[Option, String, Int] = EitherT.left(Some("error 1"))
+   * scala> val l2: EitherT[Option, String, Int] = EitherT.left(Some("error 2"))
+   * scala> val r3: EitherT[Option, String, Int] = EitherT.right(Some(3))
+   * scala> val r4: EitherT[Option, String, Int] = EitherT.right(Some(4))
+   * scala> val noneEitherT: EitherT[Option, String, Int] = EitherT.left(None)
+   *
+   * scala> l1 combine l2
+   * res0: EitherT[Option, String, Int] = EitherT(Some(Left(error 1)))
+   *
+   * scala> l1 combine r3
+   * res1: EitherT[Option, String, Int] = EitherT(Some(Left(error 1)))
+   *
+   * scala> r3 combine l1
+   * res2: EitherT[Option, String, Int] = EitherT(Some(Left(error 1)))
+   *
+   * scala> r3 combine r4
+   * res3: EitherT[Option, String, Int] = EitherT(Some(Right(7)))
+   *
+   * scala> l1 combine noneEitherT
+   * res4: EitherT[Option, String, Int] = EitherT(None)
+   *
+   * scala> noneEitherT combine l1
+   * res5: EitherT[Option, String, Int] = EitherT(None)
+   *
+   * scala> r3 combine noneEitherT
+   * res6: EitherT[Option, String, Int] = EitherT(None)
+   *
+   * scala> noneEitherT combine r4
+   * res7: EitherT[Option, String, Int] = EitherT(None)
+   * }}}
+   */
+  def combine(that: EitherT[F, A, B])(implicit F: Apply[F], B: Semigroup[B]): EitherT[F, A, B] =
+    EitherT(F.map2(this.value, that.value)(_ combine _))
+
+  def toValidated(implicit F: Functor[F]): F[Validated[A, B]] =
+    F.map(value)(_.toValidated)
+
+  def toValidatedNel(implicit F: Functor[F]): F[ValidatedNel[A, B]] =
+    F.map(value)(_.toValidatedNel)
+
+  /** Run this value as a `[[Validated]]` against the function and convert it back to an `[[EitherT]]`.
+   *
+   * The [[Applicative]] instance for `EitherT` "fails fast" - it is often useful to "momentarily" have
+   * it accumulate errors instead, which is what the `[[Validated]]` data type gives us.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> type Error = String
+   * scala> val v1: Validated[NonEmptyList[Error], Int] = Validated.Invalid(NonEmptyList.of("error 1"))
+   * scala> val v2: Validated[NonEmptyList[Error], Int] = Validated.Invalid(NonEmptyList.of("error 2"))
+   * scala> val xort: EitherT[Option, Error, Int] = EitherT(Some(Either.left("error 3")))
+   * scala> xort.withValidated { v3 => (v1 |@| v2 |@| v3.leftMap(NonEmptyList.of(_))).map{ case (i, j, k) => i + j + k } }
+   * res0: EitherT[Option, NonEmptyList[Error], Int] = EitherT(Some(Left(NonEmptyList(error 1, error 2, error 3))))
+   * }}}
+   */
+  def withValidated[AA, BB](f: Validated[A, B] => Validated[AA, BB])(implicit F: Functor[F]): EitherT[F, AA, BB] =
+    EitherT(F.map(value)(xor => f(xor.toValidated).toEither))
+
+  def show(implicit show: Show[F[Either[A, B]]]): String = show.show(value)
+
+  /**
+   * Transform this `EitherT[F, A, B]` into a `[[Nested]][F, Either[A, ?], B]`.
+   *
+   * An example where `toNested` can be used, is to get the `Apply.ap` function with the
+   * behavior from the composed `Apply` instances from `F` and `Either[A, ?]`, which is
+   * inconsistent with the behavior of the `ap` from `Monad` of `EitherT`.
+   *
+   * {{{
+   * scala> import cats.data.{Nested, EitherT}
+   * scala> import cats.implicits._
+   * scala> val ff: EitherT[List, String, Int => String] =
+   *      |   EitherT(List(Either.right(_.toString), Either.left("error")))
+   * scala> val fa: EitherT[List, String, Int] =
+   *      |   EitherT(List(Either.right(1), Either.right(2)))
+   * scala> type ErrorOr[A] = Either[String, A]
+   * scala> type ListErrorOr[A] = Nested[List, ErrorOr, A]
+   * scala> ff.ap(fa)
+   * res0: EitherT[List,String,String] = EitherT(List(Right(1), Right(2), Left(error)))
+   * scala> EitherT((ff.toNested: ListErrorOr[Int => String]).ap(fa.toNested: ListErrorOr[Int]).value)
+   * res1: EitherT[List,String,String] = EitherT(List(Right(1), Right(2), Left(error), Left(error)))
+   * }}}
+   *
+   * Note that we need the `ErrorOr` type alias above because otherwise we can't use the
+   * syntax function `ap` on `Nested[List, Either[A, ?], B]`. This won't be needed after cats has
+   * decided [[https://github.com/typelevel/cats/issues/1073 how to handle the SI-2712 fix]].
+   */
+  def toNested: Nested[F, Either[A, ?], B] = Nested[F, Either[A, ?], B](value)
+}
+
+object EitherT extends EitherTInstances with EitherTFunctions
+
+trait EitherTFunctions {
+  final def left[F[_], A, B](fa: F[A])(implicit F: Functor[F]): EitherT[F, A, B] = EitherT(F.map(fa)(Either.left))
+
+  final def right[F[_], A, B](fb: F[B])(implicit F: Functor[F]): EitherT[F, A, B] = EitherT(F.map(fb)(Either.right))
+
+  final def pure[F[_], A, B](b: B)(implicit F: Applicative[F]): EitherT[F, A, B] = right(F.pure(b))
+
+  /** Transforms an `Either` into an `EitherT`, lifted into the specified `Applicative`.
+   *
+   * Note: The return type is a FromEitherPartiallyApplied[F], which has an apply method
+   * on it, allowing you to call fromEither like this:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val t: Either[String, Int] = Either.right(3)
+   * scala> EitherT.fromEither[Option](t)
+   * res0: EitherT[Option, String, Int] = EitherT(Some(Right(3)))
+   * }}}
+   *
+   * The reason for the indirection is to emulate currying type parameters.
+   */
+  final def fromEither[F[_]]: FromEitherPartiallyApplied[F] = new FromEitherPartiallyApplied
+
+  final class FromEitherPartiallyApplied[F[_]] private[EitherTFunctions] {
+    def apply[E, A](xor: Either[E, A])(implicit F: Applicative[F]): EitherT[F, E, A] =
+      EitherT(F.pure(xor))
+  }
+}
+
+private[data] abstract class EitherTInstances extends EitherTInstances1 {
+
+  /* TODO violates right absorbtion, right distributivity, and left distributivity -- re-enable when MonadCombine laws are split in to weak/strong
+  implicit def catsDataMonadCombineForEitherT[F[_], L](implicit F: Monad[F], L: Monoid[L]): MonadCombine[EitherT[F, L, ?]] = {
+    implicit val F0 = F
+    implicit val L0 = L
+    new EitherTMonadCombine[F, L] { implicit val F = F0; implicit val L = L0 }
+  }
+  */
+
+  implicit def catsDataOrderForEitherT[F[_], L, R](implicit F: Order[F[Either[L, R]]]): Order[EitherT[F, L, R]] =
+    new EitherTOrder[F, L, R] {
+      val F0: Order[F[Either[L, R]]] = F
+    }
+
+  implicit def catsDataShowForEitherT[F[_], L, R](implicit sh: Show[F[Either[L, R]]]): Show[EitherT[F, L, R]] =
+    functor.Contravariant[Show].contramap(sh)(_.value)
+
+  implicit def catsDataBifunctorForEitherT[F[_]](implicit F: Functor[F]): Bifunctor[EitherT[F, ?, ?]] =
+    new Bifunctor[EitherT[F, ?, ?]] {
+      override def bimap[A, B, C, D](fab: EitherT[F, A, B])(f: A => C, g: B => D): EitherT[F, C, D] = fab.bimap(f, g)
+    }
+
+  implicit def catsDataTraverseForEitherT[F[_], L](implicit F: Traverse[F]): Traverse[EitherT[F, L, ?]] =
+    new EitherTTraverse[F, L] {
+      val F0: Traverse[F] = F
+    }
+
+  implicit def catsDataTransLiftForEitherT[E]: TransLift.Aux[EitherT[?[_], E, ?], Functor] =
+    new TransLift[EitherT[?[_], E, ?]] {
+      type TC[M[_]] = Functor[M]
+
+      def liftT[M[_]: Functor, A](ma: M[A]): EitherT[M, E, A] =
+        EitherT(Functor[M].map(ma)(Either.right))
+    }
+
+  implicit def catsMonoidForEitherT[F[_], L, A](implicit F: Monoid[F[Either[L, A]]]): Monoid[EitherT[F, L, A]] =
+    new EitherTMonoid[F, L, A] { implicit val F0 = F }
+
+}
+
+private[data] abstract class EitherTInstances1 extends EitherTInstances2 {
+  /* TODO violates monadFilter right empty law -- re-enable when MonadFilter laws are split in to weak/strong
+  implicit def catsDataMonadFilterForEitherT[F[_], L](implicit F: Monad[F], L: Monoid[L]): MonadFilter[EitherT[F, L, ?]] = {
+    implicit val F0 = F
+    implicit val L0 = L
+    new EitherTMonadFilter[F, L] { implicit val F = F0; implicit val L = L0 }
+  }
+   */
+
+  implicit def catsSemigroupForEitherT[F[_], L, A](implicit F: Semigroup[F[Either[L, A]]]): Semigroup[EitherT[F, L, A]] =
+    new EitherTSemigroup[F, L, A] { implicit val F0 = F }
+
+  implicit def catsDataFoldableForEitherT[F[_], L](implicit F: Foldable[F]): Foldable[EitherT[F, L, ?]] =
+    new EitherTFoldable[F, L] {
+      val F0: Foldable[F] = F
+    }
+
+  implicit def catsDataPartialOrderForEitherT[F[_], L, R](implicit F: PartialOrder[F[Either[L, R]]]): PartialOrder[EitherT[F, L, R]] =
+    new EitherTPartialOrder[F, L, R] {
+      val F0: PartialOrder[F[Either[L, R]]] = F
+    }
+
+  implicit def catsDataBitraverseForEitherT[F[_]](implicit F: Traverse[F]): Bitraverse[EitherT[F, ?, ?]] =
+    new EitherTBitraverse[F] {
+      val F0: Traverse[F] = F
+    }
+}
+
+private[data] abstract class EitherTInstances2 extends EitherTInstances3 {
+  implicit def catsDataMonadRecForEitherT[F[_], L](implicit F0: MonadRec[F]): MonadRec[EitherT[F, L, ?]] =
+    new EitherTMonadRec[F, L] { implicit val F = F0 }
+}
+
+private[data] abstract class EitherTInstances3 extends EitherTInstances4 {
+  implicit def catsDataMonadErrorForEitherT[F[_], L](implicit F0: Monad[F]): MonadError[EitherT[F, L, ?], L] =
+    new EitherTMonadError[F, L] { implicit val F = F0 }
+
+  implicit def catsDataSemigroupKForEitherT[F[_], L](implicit F0: Monad[F]): SemigroupK[EitherT[F, L, ?]] =
+    new EitherTSemigroupK[F, L] { implicit val F = F0 }
+
+  implicit def catsDataEqForEitherT[F[_], L, R](implicit F: Eq[F[Either[L, R]]]): Eq[EitherT[F, L, R]] =
+    new EitherTEq[F, L, R] {
+      val F0: Eq[F[Either[L, R]]] = F
+    }
+}
+
+private[data] abstract class EitherTInstances4 {
+  implicit def catsDataFunctorForEitherT[F[_], L](implicit F0: Functor[F]): Functor[EitherT[F, L, ?]] =
+    new EitherTFunctor[F, L] { implicit val F = F0 }
+}
+
+private[data] trait EitherTSemigroup[F[_], L, A] extends Semigroup[EitherT[F, L, A]] {
+  implicit val F0: Semigroup[F[Either[L, A]]]
+  def combine(x: EitherT[F, L , A], y: EitherT[F, L , A]): EitherT[F, L , A] =
+    EitherT(F0.combine(x.value, y.value))
+}
+
+private[data] trait EitherTMonoid[F[_], L, A] extends Monoid[EitherT[F, L, A]] with EitherTSemigroup[F, L, A] {
+  implicit val F0: Monoid[F[Either[L, A]]]
+  def empty: EitherT[F, L, A] = EitherT(F0.empty)
+}
+
+private[data] trait EitherTSemigroupK[F[_], L] extends SemigroupK[EitherT[F, L, ?]] {
+  implicit val F: Monad[F]
+  def combineK[A](x: EitherT[F, L, A], y: EitherT[F, L, A]): EitherT[F, L, A] =
+    EitherT(F.flatMap(x.value) {
+      case l @ Left(_) => y.value
+      case r @ Right(_) => F.pure(r)
+    })
+}
+
+private[data] trait EitherTFunctor[F[_], L] extends Functor[EitherT[F, L, ?]] {
+  implicit val F: Functor[F]
+  override def map[A, B](fa: EitherT[F, L, A])(f: A => B): EitherT[F, L, B] = fa map f
+}
+
+private[data] trait EitherTMonad[F[_], L] extends Monad[EitherT[F, L, ?]] with EitherTFunctor[F, L] {
+  implicit val F: Monad[F]
+  def pure[A](a: A): EitherT[F, L, A] = EitherT(F.pure(Either.right(a)))
+  def flatMap[A, B](fa: EitherT[F, L, A])(f: A => EitherT[F, L, B]): EitherT[F, L, B] = fa flatMap f
+}
+
+private[data] trait EitherTMonadError[F[_], L] extends MonadError[EitherT[F, L, ?], L] with EitherTMonad[F, L] {
+  def handleErrorWith[A](fea: EitherT[F, L, A])(f: L => EitherT[F, L, A]): EitherT[F, L, A] =
+    EitherT(F.flatMap(fea.value) {
+      case Left(e) => f(e).value
+      case r @ Right(_) => F.pure(r)
+    })
+  override def handleError[A](fea: EitherT[F, L, A])(f: L => A): EitherT[F, L, A] =
+    EitherT(F.flatMap(fea.value) {
+      case Left(e) => F.pure(Right(f(e)))
+      case r @ Right(_) => F.pure(r)
+    })
+  def raiseError[A](e: L): EitherT[F, L, A] = EitherT.left(F.pure(e))
+  override def attempt[A](fla: EitherT[F, L, A]): EitherT[F, L, Either[L, A]] = EitherT.right(fla.value)
+  override def recover[A](fla: EitherT[F, L, A])(pf: PartialFunction[L, A]): EitherT[F, L, A] =
+    fla.recover(pf)
+  override def recoverWith[A](fla: EitherT[F, L, A])(pf: PartialFunction[L, EitherT[F, L, A]]): EitherT[F, L, A] =
+    fla.recoverWith(pf)
+}
+
+private[data] trait EitherTMonadRec[F[_], L] extends MonadRec[EitherT[F, L, ?]] with EitherTMonad[F, L] {
+  implicit val F: MonadRec[F]
+  def tailRecM[A, B](a: A)(f: A => EitherT[F, L, Either[A, B]]): EitherT[F, L, B] =
+    EitherT(F.tailRecM(a)(a0 => F.map(f(a0).value){
+      case Left(l)         => Right(Left(l))
+      case Right(Left(a1)) => Left(a1)
+      case Right(Right(b)) => Right(Right(b))
+    }))
+}
+
+private[data] trait EitherTMonadFilter[F[_], L] extends MonadFilter[EitherT[F, L, ?]] with EitherTMonadError[F, L] {
+  implicit val F: Monad[F]
+  implicit val L: Monoid[L]
+  def empty[A]: EitherT[F, L, A] = EitherT(F.pure(Either.left(L.empty)))
+}
+
+/* TODO violates right absorbtion, right distributivity, and left distributivity -- re-enable when MonadCombine laws are split in to weak/strong
+private[data] trait EitherTMonadCombine[F[_], L] extends MonadCombine[EitherT[F, L, ?]] with EitherTMonadFilter[F, L] with EitherTSemigroupK[F, L] {
+  implicit val F: Monad[F]
+  implicit val L: Monoid[L]
+}
+*/
+
+private[data] sealed trait EitherTFoldable[F[_], L] extends Foldable[EitherT[F, L, ?]] {
+  implicit def F0: Foldable[F]
+
+  def foldLeft[A, B](fa: EitherT[F, L, A], b: B)(f: (B, A) => B): B =
+    fa.foldLeft(b)(f)
+
+  def foldRight[A, B](fa: EitherT[F, L, A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
+    fa.foldRight(lb)(f)
+}
+
+private[data] sealed trait EitherTTraverse[F[_], L] extends Traverse[EitherT[F, L, ?]] with EitherTFoldable[F, L] {
+  override implicit def F0: Traverse[F]
+
+  override def traverse[G[_]: Applicative, A, B](fa: EitherT[F, L, A])(f: A => G[B]): G[EitherT[F, L, B]] =
+    fa traverse f
+}
+
+private[data] sealed trait EitherTBifoldable[F[_]] extends Bifoldable[EitherT[F, ?, ?]] {
+  implicit def F0: Foldable[F]
+
+  def bifoldLeft[A, B, C](fab: EitherT[F, A, B], c: C)(f: (C, A) => C, g: (C, B) => C): C =
+    F0.foldLeft(fab.value, c)( (acc, axb) => Bifoldable[Either].bifoldLeft(axb, acc)(f, g))
+
+  def bifoldRight[A, B, C](fab: EitherT[F, A, B], c: Eval[C])(f: (A, Eval[C]) => Eval[C], g: (B, Eval[C]) => Eval[C]): Eval[C] =
+    F0.foldRight(fab.value, c)( (axb, acc) => Bifoldable[Either].bifoldRight(axb, acc)(f, g))
+}
+
+private[data] sealed trait EitherTBitraverse[F[_]] extends Bitraverse[EitherT[F, ?, ?]] with EitherTBifoldable[F] {
+  override implicit def F0: Traverse[F]
+
+  override def bitraverse[G[_], A, B, C, D](fab: EitherT[F, A, B])(f: A => G[C], g: B => G[D])(implicit G: Applicative[G]): G[EitherT[F, C, D]] =
+    fab.bitraverse(f, g)
+}
+
+private[data] sealed trait EitherTEq[F[_], L, A] extends Eq[EitherT[F, L, A]] {
+  implicit def F0: Eq[F[Either[L, A]]]
+
+  override def eqv(x: EitherT[F, L, A], y: EitherT[F, L, A]): Boolean = x === y
+}
+
+private[data] sealed trait EitherTPartialOrder[F[_], L, A] extends PartialOrder[EitherT[F, L, A]] with EitherTEq[F, L, A]{
+  override implicit def F0: PartialOrder[F[Either[L, A]]]
+
+  override def partialCompare(x: EitherT[F, L, A], y: EitherT[F, L, A]): Double =
+    x partialCompare y
+}
+
+private[data] sealed trait EitherTOrder[F[_], L, A] extends Order[EitherT[F, L, A]] with EitherTPartialOrder[F, L, A]{
+  override implicit def F0: Order[F[Either[L, A]]]
+
+  override def compare(x: EitherT[F, L, A], y: EitherT[F, L, A]): Int = x compare y
+}
diff --git a/core/src/main/scala/cats/data/Ior.scala b/core/src/main/scala/cats/data/Ior.scala
index 25cdc77474..4ff1ae39c7 100644
--- a/core/src/main/scala/cats/data/Ior.scala
+++ b/core/src/main/scala/cats/data/Ior.scala
@@ -10,14 +10,14 @@ import cats.functor.Bifunctor
  *  - `[[Ior.Right Right]][B]`
  *  - `[[Ior.Both Both]][A, B]`
  *
- * `A [[Ior]] B` is similar to `A [[Xor]] B`, except that it can represent the simultaneous presence of
- * an `A` and a `B`. It is right-biased like [[Xor]], so methods such as `map` and `flatMap` operate on the
+ * `A [[Ior]] B` is similar to `Either[A, B]`, except that it can represent the simultaneous presence of
+ * an `A` and a `B`. It is right-biased so methods such as `map` and `flatMap` operate on the
  * `B` value. Some methods, like `flatMap`, handle the presence of two [[Ior.Both Both]] values using a
- * `[[Semigroup]][A]`, while other methods, like [[toXor]], ignore the `A` value in a [[Ior.Both Both]].
+ * `[[Semigroup]][A]`, while other methods, like [[toEither]], ignore the `A` value in a [[Ior.Both Both]].
  *
- * `A [[Ior]] B` is isomorphic to `(A [[Xor]] B) [[Xor]] (A, B)`, but provides methods biased toward `B`
+ * `A [[Ior]] B` is isomorphic to `Either[Either[A, B], (A, B)]`, but provides methods biased toward `B`
  * values, regardless of whether the `B` values appear in a [[Ior.Right Right]] or a [[Ior.Both Both]].
- * The isomorphic [[Xor]] form can be accessed via the [[unwrap]] method.
+ * The isomorphic [[Either]] form can be accessed via the [[unwrap]] method.
  */
 sealed abstract class Ior[+A, +B] extends Product with Serializable {
 
@@ -35,10 +35,10 @@ sealed abstract class Ior[+A, +B] extends Product with Serializable {
   final def right: Option[B] = fold(_ => None, b => Some(b), (_, b) => Some(b))
   final def onlyLeft: Option[A] = fold(a => Some(a), _ => None, (_, _) => None)
   final def onlyRight: Option[B] = fold(_ => None, b => Some(b), (_, _) => None)
-  final def onlyLeftOrRight: Option[A Xor B] = fold(a => Some(Xor.left(a)), b => Some(Xor.right(b)), (_, _) => None)
+  final def onlyLeftOrRight: Option[Either[A, B]] = fold(a => Some(Left(a)), b => Some(Right(b)), (_, _) => None)
   final def onlyBoth: Option[(A, B)] = fold(_ => None, _ => None, (a, b) => Some((a, b)))
   final def pad: (Option[A], Option[B]) = fold(a => (Some(a), None), b => (None, Some(b)), (a, b) => (Some(a), Some(b)))
-  final def unwrap: (A Xor B) Xor (A, B) = fold(a => Xor.left(Xor.left(a)), b => Xor.left(Xor.right(b)), (a, b) => Xor.right((a, b)))
+  final def unwrap: Either[Either[A, B], (A, B)] = fold(a => Left(Left(a)), b => Left(Right(b)), (a, b) => Right((a, b)))
 
   final def toXor: A Xor B = fold(Xor.left, Xor.right, (_, b) => Xor.right(b))
   final def toEither: Either[A, B] = fold(Left(_), Right(_), (_, b) => Right(b))
diff --git a/core/src/main/scala/cats/data/Kleisli.scala b/core/src/main/scala/cats/data/Kleisli.scala
index 72d509d050..1d4eb2aa93 100644
--- a/core/src/main/scala/cats/data/Kleisli.scala
+++ b/core/src/main/scala/cats/data/Kleisli.scala
@@ -101,7 +101,7 @@ private[data] sealed abstract class KleisliInstances extends KleisliInstances0 {
     new Choice[Kleisli[F, ?, ?]] {
       def id[A]: Kleisli[F, A, A] = Kleisli(ev.pure(_))
 
-      def choice[A, B, C](f: Kleisli[F, A, C], g: Kleisli[F, B, C]): Kleisli[F, Xor[A, B], C] =
+      def choice[A, B, C](f: Kleisli[F, A, C], g: Kleisli[F, B, C]): Kleisli[F, Either[A, B], C] =
         Kleisli(_.fold(f.run, g.run))
 
       def compose[A, B, C](f: Kleisli[F, B, C], g: Kleisli[F, A, B]): Kleisli[F, A, C] =
diff --git a/core/src/main/scala/cats/data/NonEmptyList.scala b/core/src/main/scala/cats/data/NonEmptyList.scala
index 8e70badd01..be0f5a6157 100644
--- a/core/src/main/scala/cats/data/NonEmptyList.scala
+++ b/core/src/main/scala/cats/data/NonEmptyList.scala
@@ -181,16 +181,16 @@ private[data] sealed trait NonEmptyListInstances extends NonEmptyListInstances0
       override def foldRight[A, B](fa: NonEmptyList[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
         fa.foldRight(lb)(f)
 
-      def tailRecM[A, B](a: A)(f: A => NonEmptyList[A Xor B]): NonEmptyList[B] = {
+      def tailRecM[A, B](a: A)(f: A => NonEmptyList[Either[A, B]]): NonEmptyList[B] = {
         val buf = new ListBuffer[B]
-        @tailrec def go(v: NonEmptyList[A Xor B]): Unit = v.head match {
-            case Xor.Right(b) =>
+        @tailrec def go(v: NonEmptyList[Either[A, B]]): Unit = v.head match {
+            case Right(b) =>
             buf += b
             NonEmptyList.fromList(v.tail) match {
               case Some(t) => go(t)
               case None => ()
             }
-          case Xor.Left(a) => go(f(a) ++ v.tail)
+          case Left(a) => go(f(a) ++ v.tail)
           }
         go(f(a))
         NonEmptyList.fromListUnsafe(buf.result())
diff --git a/core/src/main/scala/cats/data/NonEmptyVector.scala b/core/src/main/scala/cats/data/NonEmptyVector.scala
index d80754153e..8727e54a1a 100644
--- a/core/src/main/scala/cats/data/NonEmptyVector.scala
+++ b/core/src/main/scala/cats/data/NonEmptyVector.scala
@@ -182,16 +182,16 @@ private[data] sealed trait NonEmptyVectorInstances {
       override def foldRight[A, B](fa: NonEmptyVector[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
         fa.foldRight(lb)(f)
 
-      def tailRecM[A, B](a: A)(f: A => NonEmptyVector[A Xor B]): NonEmptyVector[B] = {
+      def tailRecM[A, B](a: A)(f: A => NonEmptyVector[Either[A, B]]): NonEmptyVector[B] = {
         val buf = new VectorBuilder[B]
-        @tailrec def go(v: NonEmptyVector[A Xor B]): Unit = v.head match {
-            case Xor.Right(b) =>
+        @tailrec def go(v: NonEmptyVector[Either[A, B]]): Unit = v.head match {
+            case Right(b) =>
             buf += b
             NonEmptyVector.fromVector(v.tail) match {
               case Some(t) => go(t)
               case None => ()
             }
-          case Xor.Left(a) => go(f(a).concat(v.tail))
+          case Left(a) => go(f(a).concat(v.tail))
           }
         go(f(a))
         NonEmptyVector.fromVectorUnsafe(buf.result())
diff --git a/core/src/main/scala/cats/data/OptionT.scala b/core/src/main/scala/cats/data/OptionT.scala
index 6f49a7f35f..7dcd216f3f 100644
--- a/core/src/main/scala/cats/data/OptionT.scala
+++ b/core/src/main/scala/cats/data/OptionT.scala
@@ -1,7 +1,8 @@
 package cats
 package data
 
-import instances.option.{catsStdInstancesForOption => optionInstance}
+import cats.instances.option.{catsStdInstancesForOption => optionInstance}
+import cats.syntax.either._
 
 /**
  * `OptionT[F[_], A]` is a light wrapper on an `F[Option[A]]` with some
@@ -85,11 +86,11 @@ final case class OptionT[F[_], A](value: F[Option[A]]) {
         case None => default
       })
 
-  def toRight[L](left: => L)(implicit F: Functor[F]): XorT[F, L, A] =
-    XorT(cata(Xor.Left(left), Xor.Right.apply))
+  def toRight[L](left: => L)(implicit F: Functor[F]): EitherT[F, L, A] =
+    EitherT(cata(Left(left), Right.apply))
 
-  def toLeft[R](right: => R)(implicit F: Functor[F]): XorT[F, A, R] =
-    XorT(cata(Xor.Right(right), Xor.Left.apply))
+  def toLeft[R](right: => R)(implicit F: Functor[F]): EitherT[F, A, R] =
+    EitherT(cata(Right(right), Left.apply))
 
   def show(implicit F: Show[F[Option[A]]]): String = F.show(value)
 
@@ -256,9 +257,9 @@ private[data] trait OptionTMonad[F[_]] extends Monad[OptionT[F, ?]] {
 private[data] trait OptionTMonadRec[F[_]] extends MonadRec[OptionT[F, ?]] with OptionTMonad[F] {
   implicit def F: MonadRec[F]
 
-  def tailRecM[A, B](a: A)(f: A => OptionT[F, A Xor B]): OptionT[F, B] =
+  def tailRecM[A, B](a: A)(f: A => OptionT[F, Either[A, B]]): OptionT[F, B] =
     OptionT(F.tailRecM(a)(a0 => F.map(f(a0).value)(
-      _.fold(Xor.right[A, Option[B]](None))(_.map(Some(_)))
+      _.fold(Either.right[A, Option[B]](None))(_.map(b => Some(b): Option[B]))
     )))
 }
 
diff --git a/core/src/main/scala/cats/data/StateT.scala b/core/src/main/scala/cats/data/StateT.scala
index 13799da11f..ab2fdb4525 100644
--- a/core/src/main/scala/cats/data/StateT.scala
+++ b/core/src/main/scala/cats/data/StateT.scala
@@ -1,6 +1,8 @@
 package cats
 package data
 
+import cats.syntax.either._
+
 /**
  * `StateT[F, S, A]` is similar to `Kleisli[F, S, A]` in that it takes an `S`
  * argument and produces an `A` value wrapped in `F`. However, it also produces
@@ -230,7 +232,7 @@ private[data] sealed trait StateTMonadState[F[_], S] extends MonadState[StateT[F
 private[data] sealed trait StateTMonadRec[F[_], S] extends MonadRec[StateT[F, S, ?]] with StateTMonad[F, S] {
   override implicit def F: MonadRec[F]
 
-  def tailRecM[A, B](a: A)(f: A => StateT[F, S, A Xor B]): StateT[F, S, B] =
+  def tailRecM[A, B](a: A)(f: A => StateT[F, S, Either[A, B]]): StateT[F, S, B] =
     StateT[F, S, B](s => F.tailRecM[(S, A), (S, B)]((s, a)) {
       case (s, a) => F.map(f(a).run(s)) { case (s, ab) => ab.bimap((s, _), (s, _)) }
     })
diff --git a/core/src/main/scala/cats/data/Validated.scala b/core/src/main/scala/cats/data/Validated.scala
index cb7cedd3b2..b073ed0e7e 100644
--- a/core/src/main/scala/cats/data/Validated.scala
+++ b/core/src/main/scala/cats/data/Validated.scala
@@ -80,11 +80,11 @@ sealed abstract class Validated[+E, +A] extends Product with Serializable {
   def toXor: Xor[E, A] = fold(Xor.Left.apply, Xor.Right.apply)
 
   /**
-   * Convert to an Xor, apply a function, convert back.  This is handy
-   * when you want to use the Monadic properties of the Xor type.
+   * Convert to an Either, apply a function, convert back.  This is handy
+   * when you want to use the Monadic properties of the Either type.
    */
-  def withXor[EE, B](f: (E Xor A) => (EE Xor B)): Validated[EE, B] =
-    f(toXor).toValidated
+  def withEither[EE, B](f: Either[E, A] => Either[EE, B]): Validated[EE, B] =
+    Validated.fromEither(f(toEither))
 
   /**
    * Validated is a [[functor.Bifunctor]], this method applies one of the
@@ -177,7 +177,7 @@ sealed abstract class Validated[+E, +A] extends Product with Serializable {
    * This allows "chained" validation: the output of one validation can be fed
    * into another validation function.
    *
-   * This function is similar to `Xor.flatMap`. It's not called `flatMap`,
+   * This function is similar to `flatMap` on `Either`. It's not called `flatMap`,
    * because by Cats convention, `flatMap` is a monadic bind that is consistent
    * with `ap`. This method is not consistent with [[ap]] (or other
    * `Apply`-based methods), because it has "fail-fast" behavior as opposed to
diff --git a/core/src/main/scala/cats/data/Xor.scala b/core/src/main/scala/cats/data/Xor.scala
index b7e599063d..56fa4a5055 100644
--- a/core/src/main/scala/cats/data/Xor.scala
+++ b/core/src/main/scala/cats/data/Xor.scala
@@ -263,11 +263,11 @@ private[data] sealed abstract class XorInstances extends XorInstances1 {
       def flatMap[B, C](fa: A Xor B)(f: B => A Xor C): A Xor C = fa.flatMap(f)
       override def ap[B, C](x: A Xor (B => C))(y: A Xor B): A Xor C = y.ap(x)
       def pure[B](b: B): A Xor B = Xor.right(b)
-      @tailrec def tailRecM[B, C](b: B)(f: B => A Xor (B Xor C)): A Xor C =
+      @tailrec def tailRecM[B, C](b: B)(f: B => A Xor Either[B, C]): A Xor C =
         f(b) match {
           case Xor.Left(a) => Xor.Left(a)
-          case Xor.Right(Xor.Left(b1)) => tailRecM(b1)(f)
-          case Xor.Right(Xor.Right(c)) => Xor.Right(c)
+          case Xor.Right(Left(b1)) => tailRecM(b1)(f)
+          case Xor.Right(Right(c)) => Xor.Right(c)
         }
       def handleErrorWith[B](fea: Xor[A, B])(f: A => Xor[A, B]): Xor[A, B] =
         fea match {
@@ -278,7 +278,7 @@ private[data] sealed abstract class XorInstances extends XorInstances1 {
       override def map[B, C](fa: A Xor B)(f: B => C): A Xor C = fa.map(f)
       override def map2Eval[B, C, Z](fb: A Xor B, fc: Eval[A Xor C])(f: (B, C) => Z): Eval[A Xor Z] =
         fb.map2Eval(fc)(f)
-      override def attempt[B](fab: A Xor B): A Xor (A Xor B) = Xor.right(fab)
+      override def attempt[B](fab: A Xor B): A Xor (Either[A, B]) = Xor.right(fab.toEither)
       override def recover[B](fab: A Xor B)(pf: PartialFunction[A, B]): A Xor B =
         fab recover pf
       override def recoverWith[B](fab: A Xor B)(pf: PartialFunction[A, A Xor B]): A Xor B =
diff --git a/core/src/main/scala/cats/data/XorT.scala b/core/src/main/scala/cats/data/XorT.scala
index 5e6a44a5fa..808d081f0a 100644
--- a/core/src/main/scala/cats/data/XorT.scala
+++ b/core/src/main/scala/cats/data/XorT.scala
@@ -55,6 +55,8 @@ final case class XorT[F[_], A, B](value: F[A Xor B]) {
 
   def toEither(implicit F: Functor[F]): F[Either[A, B]] = F.map(value)(_.toEither)
 
+  def toEitherT(implicit F: Functor[F]): EitherT[F, A, B] = EitherT(toEither)
+
   def toOption(implicit F: Functor[F]): OptionT[F, B] = OptionT(F.map(value)(_.toOption))
 
   def to[G[_]](implicit F: Functor[F], G: Alternative[G]): F[G[B]] =
@@ -382,7 +384,7 @@ private[data] trait XorTMonadError[F[_], L] extends MonadError[XorT[F, L, ?], L]
       case r @ Xor.Right(_) => F.pure(r)
     })
   def raiseError[A](e: L): XorT[F, L, A] = XorT.left(F.pure(e))
-  override def attempt[A](fla: XorT[F, L, A]): XorT[F, L, L Xor A] = XorT.right(fla.value)
+  override def attempt[A](fla: XorT[F, L, A]): XorT[F, L, Either[L, A]] = XorT.right(fla.toEither)
   override def recover[A](fla: XorT[F, L, A])(pf: PartialFunction[L, A]): XorT[F, L, A] =
     fla.recover(pf)
   override def recoverWith[A](fla: XorT[F, L, A])(pf: PartialFunction[L, XorT[F, L, A]]): XorT[F, L, A] =
@@ -391,11 +393,11 @@ private[data] trait XorTMonadError[F[_], L] extends MonadError[XorT[F, L, ?], L]
 
 private[data] trait XorTMonadRec[F[_], L] extends MonadRec[XorT[F, L, ?]] with XorTMonad[F, L] {
   implicit val F: MonadRec[F]
-  def tailRecM[A, B](a: A)(f: A => XorT[F, L, A Xor B]): XorT[F, L, B] =
+  def tailRecM[A, B](a: A)(f: A => XorT[F, L, Either[A, B]]): XorT[F, L, B] =
     XorT(F.tailRecM(a)(a0 => F.map(f(a0).value){
-      case Xor.Left(l) => Xor.Right(Xor.Left(l))
-      case Xor.Right(Xor.Left(a1)) => Xor.Left(a1)
-      case Xor.Right(Xor.Right(b)) => Xor.Right(Xor.Right(b))
+      case Xor.Left(l)         => Right(Xor.Left(l))
+      case Xor.Right(Left(a1)) => Left(a1)
+      case Xor.Right(Right(b)) => Right(Xor.Right(b))
     }))
 }
 
diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
index 8bdb39dd64..21c20926cb 100644
--- a/core/src/main/scala/cats/instances/either.scala
+++ b/core/src/main/scala/cats/instances/either.scala
@@ -1,8 +1,8 @@
 package cats
 package instances
 
+import cats.syntax.either._
 import scala.annotation.tailrec
-import cats.data.Xor
 
 trait EitherInstances extends EitherInstances1 {
   implicit val catsStdBitraverseForEither: Bitraverse[Either] =
@@ -26,22 +26,29 @@ trait EitherInstances extends EitherInstances1 {
         }
     }
 
-  implicit def catsStdInstancesForEither[A]: MonadRec[Either[A, ?]] with Traverse[Either[A, ?]] =
-    new MonadRec[Either[A, ?]] with Traverse[Either[A, ?]] {
+  implicit def catsStdInstancesForEither[A]: MonadRec[Either[A, ?]] with MonadError[Either[A, ?], A] with Traverse[Either[A, ?]] =
+    new MonadRec[Either[A, ?]] with MonadError[Either[A, ?], A] with Traverse[Either[A, ?]] {
       def pure[B](b: B): Either[A, B] = Right(b)
 
       def flatMap[B, C](fa: Either[A, B])(f: B => Either[A, C]): Either[A, C] =
         fa.right.flatMap(f)
 
+      def handleErrorWith[B](fea: Either[A, B])(f: A => Either[A, B]): Either[A, B] =
+        fea match {
+          case Left(e) => f(e)
+          case r @ Right(_) => r
+        }
+      def raiseError[B](e: A): Either[A, B] = Left(e)
+
       override def map[B, C](fa: Either[A, B])(f: B => C): Either[A, C] =
         fa.right.map(f)
 
       @tailrec
-      def tailRecM[B, C](b: B)(f: B => Either[A, B Xor C]): Either[A, C] =
+      def tailRecM[B, C](b: B)(f: B => Either[A, Either[B, C]]): Either[A, C] =
         f(b) match {
-          case Left(a) => Left(a)
-          case Right(Xor.Left(b1)) => tailRecM(b1)(f)
-          case Right(Xor.Right(c)) => Right(c)
+          case Left(a)         => Left(a)
+          case Right(Left(b1)) => tailRecM(b1)(f)
+          case Right(Right(c)) => Right(c)
         }
 
       override def map2Eval[B, C, Z](fb: Either[A, B], fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
@@ -64,6 +71,14 @@ trait EitherInstances extends EitherInstances1 {
 
       def foldRight[B, C](fa: Either[A, B], lc: Eval[C])(f: (B, Eval[C]) => Eval[C]): Eval[C] =
         fa.fold(_ => lc, b => f(b, lc))
+
+      override def attempt[B](fab: Either[A, B]): Either[A, Either[A, B]] = Right(fab)
+      override def recover[B](fab: Either[A, B])(pf: PartialFunction[A, B]): Either[A, B] =
+        fab recover pf
+      override def recoverWith[B](fab: Either[A, B])(pf: PartialFunction[A, Either[A, B]]): Either[A, B] =
+        fab recoverWith pf
+      override def ensure[B](fab: Either[A, B])(error: => A)(predicate: B => Boolean): Either[A, B] =
+        fab.ensure(error)(predicate)
     }
 
   implicit def catsStdOrderForEither[A, B](implicit A: Order[A], B: Order[B]): Order[Either[A, B]] = new Order[Either[A, B]] {
diff --git a/core/src/main/scala/cats/instances/function.scala b/core/src/main/scala/cats/instances/function.scala
index 758d620b80..3444518029 100644
--- a/core/src/main/scala/cats/instances/function.scala
+++ b/core/src/main/scala/cats/instances/function.scala
@@ -2,7 +2,6 @@ package cats
 package instances
 
 import cats.arrow.{Arrow, Choice}
-import cats.data.Xor
 import cats.functor.Contravariant
 
 private[instances] sealed trait Function0Instances {
@@ -50,10 +49,10 @@ private[instances] sealed trait Function1Instances extends Function1Instances0 {
 
   implicit val catsStdInstancesForFunction1: Choice[Function1] with Arrow[Function1] =
     new Choice[Function1] with Arrow[Function1] {
-      def choice[A, B, C](f: A => C, g: B => C): Xor[A, B] => C =
+      def choice[A, B, C](f: A => C, g: B => C): Either[A, B] => C =
         _ match {
-          case Xor.Left(a) => f(a)
-          case Xor.Right(b) => g(b)
+          case Left(a)  => f(a)
+          case Right(b) => g(b)
         }
 
       def lift[A, B](f: A => B): A => B = f
diff --git a/core/src/main/scala/cats/instances/future.scala b/core/src/main/scala/cats/instances/future.scala
index a7d444acf1..b6796e8d46 100644
--- a/core/src/main/scala/cats/instances/future.scala
+++ b/core/src/main/scala/cats/instances/future.scala
@@ -1,8 +1,6 @@
 package cats
 package instances
 
-import cats.data.Xor
-
 import scala.util.control.NonFatal
 import scala.concurrent.{ExecutionContext, Future}
 
@@ -18,10 +16,10 @@ trait FutureInstances extends FutureInstances1 {
        * Note that while this implementation will not compile with `@tailrec`,
        * it is in fact stack-safe.
        */
-      final def tailRecM[B, C](b: B)(f: B => Future[(B Xor C)]): Future[C] =
+      final def tailRecM[B, C](b: B)(f: B => Future[Either[B, C]]): Future[C] =
         f(b).flatMap {
-          case Xor.Left(b1) => tailRecM(b1)(f)
-          case Xor.Right(c) => Future.successful(c)
+          case Left(b1) => tailRecM(b1)(f)
+          case Right(c) => Future.successful(c)
         }
 
       def handleErrorWith[A](fea: Future[A])(f: Throwable => Future[A]): Future[A] = fea.recoverWith { case t => f(t) }
@@ -29,8 +27,8 @@ trait FutureInstances extends FutureInstances1 {
       def raiseError[A](e: Throwable): Future[A] = Future.failed(e)
       override def handleError[A](fea: Future[A])(f: Throwable => A): Future[A] = fea.recover { case t => f(t) }
 
-      override def attempt[A](fa: Future[A]): Future[Throwable Xor A] =
-        (fa map Xor.right) recover { case NonFatal(t) => Xor.left(t) }
+      override def attempt[A](fa: Future[A]): Future[Either[Throwable, A]] =
+        (fa.map(a => Right[Throwable, A](a))) recover { case NonFatal(t) => Left(t) }
 
       override def recover[A](fa: Future[A])(pf: PartialFunction[Throwable, A]): Future[A] = fa.recover(pf)
 
diff --git a/core/src/main/scala/cats/instances/list.scala b/core/src/main/scala/cats/instances/list.scala
index 9b38b6af21..7e83c95b91 100644
--- a/core/src/main/scala/cats/instances/list.scala
+++ b/core/src/main/scala/cats/instances/list.scala
@@ -6,8 +6,6 @@ import cats.syntax.show._
 import scala.annotation.tailrec
 import scala.collection.mutable.ListBuffer
 
-import cats.data.Xor
-
 trait ListInstances extends cats.kernel.instances.ListInstances {
 
   implicit val catsStdInstancesForList: TraverseFilter[List] with MonadCombine[List] with MonadRec[List] with CoflatMap[List] =
@@ -28,12 +26,12 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
       override def map2[A, B, Z](fa: List[A], fb: List[B])(f: (A, B) => Z): List[Z] =
         fa.flatMap(a => fb.map(b => f(a, b)))
 
-      def tailRecM[A, B](a: A)(f: A => List[A Xor B]): List[B] = {
+      def tailRecM[A, B](a: A)(f: A => List[Either[A, B]]): List[B] = {
         val buf = List.newBuilder[B]
-        @tailrec def go(lists: List[List[A Xor B]]): Unit = lists match {
+        @tailrec def go(lists: List[List[Either[A, B]]]): Unit = lists match {
           case (ab :: abs) :: tail => ab match {
-            case Xor.Right(b) => buf += b; go(abs :: tail)
-            case Xor.Left(a) => go(f(a) :: abs :: tail)
+            case Right(b) => buf += b; go(abs :: tail)
+            case Left(a) => go(f(a) :: abs :: tail)
           }
           case Nil :: tail => go(tail)
           case Nil => ()
diff --git a/core/src/main/scala/cats/instances/option.scala b/core/src/main/scala/cats/instances/option.scala
index 7ce5d7ffc9..d77d4ef81e 100644
--- a/core/src/main/scala/cats/instances/option.scala
+++ b/core/src/main/scala/cats/instances/option.scala
@@ -2,7 +2,6 @@ package cats
 package instances
 
 import scala.annotation.tailrec
-import cats.data.Xor
 
 trait OptionInstances extends cats.kernel.instances.OptionInstances {
 
@@ -22,11 +21,11 @@ trait OptionInstances extends cats.kernel.instances.OptionInstances {
         fa.flatMap(f)
 
       @tailrec
-      def tailRecM[A, B](a: A)(f: A => Option[A Xor B]): Option[B] =
+      def tailRecM[A, B](a: A)(f: A => Option[Either[A, B]]): Option[B] =
         f(a) match {
           case None => None
-          case Some(Xor.Left(a1)) => tailRecM(a1)(f)
-          case Some(Xor.Right(b)) => Some(b)
+          case Some(Left(a1)) => tailRecM(a1)(f)
+          case Some(Right(b)) => Some(b)
         }
 
       override def map2[A, B, Z](fa: Option[A], fb: Option[B])(f: (A, B) => Z): Option[Z] =
diff --git a/core/src/main/scala/cats/instances/try.scala b/core/src/main/scala/cats/instances/try.scala
index e92dcdacd7..b87f808126 100644
--- a/core/src/main/scala/cats/instances/try.scala
+++ b/core/src/main/scala/cats/instances/try.scala
@@ -1,7 +1,6 @@
 package cats
 package instances
 
-import cats.data.Xor
 import TryInstances.castFailure
 
 import scala.util.control.NonFatal
@@ -53,11 +52,11 @@ trait TryInstances extends TryInstances1 {
           case f: Failure[_] => G.pure(castFailure[B](f))
         }
 
-      @tailrec final def tailRecM[B, C](b: B)(f: B => Try[(B Xor C)]): Try[C] =
+      @tailrec final def tailRecM[B, C](b: B)(f: B => Try[Either[B, C]]): Try[C] =
         f(b) match {
           case f: Failure[_] => castFailure[C](f)
-          case Success(Xor.Left(b1)) => tailRecM(b1)(f)
-          case Success(Xor.Right(c)) => Success(c)
+          case Success(Left(b1)) => tailRecM(b1)(f)
+          case Success(Right(c)) => Success(c)
         }
 
       def handleErrorWith[A](ta: Try[A])(f: Throwable => Try[A]): Try[A] =
@@ -67,8 +66,8 @@ trait TryInstances extends TryInstances1 {
       override def handleError[A](ta: Try[A])(f: Throwable => A): Try[A] =
         ta.recover { case t => f(t) }
 
-      override def attempt[A](ta: Try[A]): Try[Throwable Xor A] =
-        (ta map Xor.right) recover { case NonFatal(t) => Xor.left(t) }
+      override def attempt[A](ta: Try[A]): Try[Either[Throwable, A]] =
+        (ta.map(a => Right[Throwable, A](a))) recover { case NonFatal(t) => Left(t) }
 
       override def recover[A](ta: Try[A])(pf: PartialFunction[Throwable, A]): Try[A] =
         ta.recover(pf)
diff --git a/core/src/main/scala/cats/package.scala b/core/src/main/scala/cats/package.scala
index 5ab9678a10..7dd5f7ca03 100644
--- a/core/src/main/scala/cats/package.scala
+++ b/core/src/main/scala/cats/package.scala
@@ -1,5 +1,4 @@
 import scala.annotation.tailrec
-import cats.data.Xor
 
 /**
  * Symbolic aliases for various types are defined here.
@@ -34,9 +33,9 @@ package object cats {
       def extract[A](a: A): A = a
       def flatMap[A, B](a: A)(f: A => B): B = f(a)
       def coflatMap[A, B](a: A)(f: A => B): B = f(a)
-      @tailrec def tailRecM[A, B](a: A)(f: A => A Xor B): B = f(a) match {
-        case Xor.Left(a1) => tailRecM(a1)(f)
-        case Xor.Right(b) => b
+      @tailrec def tailRecM[A, B](a: A)(f: A => Either[A, B]): B = f(a) match {
+        case Left(a1) => tailRecM(a1)(f)
+        case Right(b) => b
       }
       override def map[A, B](fa: A)(f: A => B): B = f(fa)
       override def ap[A, B](ff: A => B)(fa: A): B = ff(fa)
diff --git a/core/src/main/scala/cats/syntax/applicativeError.scala b/core/src/main/scala/cats/syntax/applicativeError.scala
index 73c57aef64..10dc9a6079 100644
--- a/core/src/main/scala/cats/syntax/applicativeError.scala
+++ b/core/src/main/scala/cats/syntax/applicativeError.scala
@@ -1,7 +1,7 @@
 package cats
 package syntax
 
-import cats.data.{Xor, XorT}
+import cats.data.EitherT
 
 trait ApplicativeErrorSyntax {
   implicit def catsSyntaxApplicativeErrorId[E](e: E): ApplicativeErrorIdOps[E] =
@@ -24,10 +24,10 @@ final class ApplicativeErrorOps[F[_], E, A](fa: F[A])(implicit F: ApplicativeErr
   def handleErrorWith(f: E => F[A]): F[A] =
     F.handleErrorWith(fa)(f)
 
-  def attempt: F[E Xor A] =
+  def attempt: F[Either[E, A]] =
     F.attempt(fa)
 
-  def attemptT: XorT[F, E, A] =
+  def attemptT: EitherT[F, E, A] =
     F.attemptT(fa)
 
   def recover(pf: PartialFunction[E, A]): F[A] =
diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index b91efb2635..fb728c2860 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -1,13 +1,163 @@
 package cats
 package syntax
 
-import cats.data.Xor
+import cats.data.{Ior, Validated, ValidatedNel, Xor}
+import scala.reflect.ClassTag
+import scala.util.{Failure, Success, Try}
 
 trait EitherSyntax {
   implicit def catsSyntaxEither[A, B](eab: Either[A, B]): EitherOps[A, B] = new EitherOps(eab)
+
+  implicit def catsSyntaxEitherObject(either: Either.type): EitherObjectOps = new EitherObjectOps(either)
 }
 
 final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
+  def foreach(f: B => Unit): Unit = eab.fold(_ => (), f)
+
+  def getOrElse[BB >: B](default: => BB): BB = eab.fold(_ => default, identity)
+
+  def orElse[C, BB >: B](fallback: => Either[C, BB]): Either[C, BB]= eab match {
+    case Left(_)      => fallback
+    case r @ Right(_) => r.asInstanceOf[Either[C, BB]]
+  }
+
+  def recover[BB >: B](pf: PartialFunction[A, BB]): Either[A, BB] = eab match {
+    case Left(a) if pf.isDefinedAt(a) => Right(pf(a))
+    case _                            => eab
+  }
+
+  def recoverWith[AA >: A, BB >: B](pf: PartialFunction[A, Either[AA, BB]]): Either[AA, BB] = eab match {
+    case Left(a) if pf.isDefinedAt(a) => pf(a)
+    case _                            => eab
+  }
+
+  def valueOr[BB >: B](f: A => BB): BB = eab.fold(f, identity)
+
+  def forall(f: B => Boolean): Boolean = eab.fold(_ => true, f)
+
+  def exists(f: B => Boolean): Boolean = eab.fold(_ => false, f)
+
+  def ensure[AA >: A](onFailure: => AA)(f: B => Boolean): Either[AA, B] =
+    eab.fold(_ => eab, b => if (f(b)) eab else Left(onFailure))
+
+  def toIor: A Ior B = eab.fold(Ior.left, Ior.right)
+
+  def toOption: Option[B] = eab.fold(_ => None, Some(_))
+
+  def toList: List[B] = eab.fold(_ => Nil, _ :: Nil)
+
+  def toTry(implicit ev: A <:< Throwable): Try[B] = eab.fold(a => Failure(ev(a)), Success(_))
+
+  def toValidated: Validated[A, B] = eab.fold(Validated.Invalid.apply, Validated.Valid.apply)
+
+  /** Returns a [[ValidatedNel]] representation of this disjunction with the `Left` value
+   * as a single element on the `Invalid` side of the [[NonEmptyList]]. */
+  def toValidatedNel[AA >: A]: ValidatedNel[AA, B] = eab.fold(Validated.invalidNel, Validated.valid)
+
+  def withValidated[AA, BB](f: Validated[A, B] => Validated[AA, BB]): Either[AA, BB] =
+    f(toValidated).toEither
+
+  def to[F[_], BB >: B](implicit F: Alternative[F]): F[BB] =
+    eab.fold(_ => F.empty, F.pure)
+
+  def bimap[C, D](fa: A => C, fb: B => D): Either[C, D] = eab match {
+    case Left(a)  => Left(fa(a))
+    case Right(b) => Right(fb(b))
+  }
+
+  def map[C](f: B => C): Either[A, C] = eab match {
+    case l @ Left(_) => l.asInstanceOf[Either[A, C]]
+    case Right(b)    => Right(f(b))
+  }
+
+  def map2Eval[AA >: A, C, Z](fc: Eval[Either[AA, C]])(f: (B, C) => Z): Eval[Either[AA, Z]] =
+    eab match {
+      case l @ Left(_) => Now(l.asInstanceOf[Either[AA, Z]])
+      case Right(b)    => fc.map(either => new EitherOps(either).map(f(b, _)))
+    }
+
+  def leftMap[C](f: A => C): Either[C, B] = eab match {
+    case Left(a)      => Left(f(a))
+    case r @ Right(_) => r.asInstanceOf[Either[C, B]]
+  }
+
+  def flatMap[AA >: A, D](f: B => Either[AA, D]): Either[AA, D] = eab match {
+    case l @ Left(_) => l.asInstanceOf[Either[AA, D]]
+    case Right(b)    => f(b)
+  }
+
+  def compare[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: Order[AA], BB: Order[BB]): Int = eab.fold(
+    a => that.fold(AA.compare(a, _), _ => -1),
+    b => that.fold(_ => 1, BB.compare(b, _))
+  )
+
+  def partialCompare[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: PartialOrder[AA], BB: PartialOrder[BB]): Double =
+    eab.fold(
+      a => that.fold(AA.partialCompare(a, _), _ => -1),
+      b => that.fold(_ => 1, BB.partialCompare(b, _))
+    )
+
+  def ===[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: Eq[AA], BB: Eq[BB]): Boolean = eab.fold(
+    a => that.fold(AA.eqv(a, _), _ => false),
+    b => that.fold(_ => false, BB.eqv(b, _))
+  )
+
+  def traverse[F[_], AA >: A, D](f: B => F[D])(implicit F: Applicative[F]): F[Either[AA, D]] = eab match {
+    case l @ Left(_) => F.pure(l.asInstanceOf[Either[AA, D]])
+    case Right(b) => F.map(f(b))(Right(_))
+  }
+
+  def foldLeft[C](c: C)(f: (C, B) => C): C = eab.fold(_ => c, f(c, _))
+
+  def foldRight[C](lc: Eval[C])(f: (B, Eval[C]) => Eval[C]): Eval[C] =
+    eab.fold(_ => lc, b => f(b, lc))
+
+  /**
+   * Combine with another `Either` value.
+   *
+   * If this `Either` is a `Left` then it will be returned as-is.
+   * If this `Either` is a `Right` and `that` `Either` is a left, then `that` will be
+   * returned.
+   * If both `Either`s are `Right`s, then the `Semigroup[BB]` instance will be used
+   * to combine both values and return them as a `Right`.
+   * Note: If both `Either`s are `Left`s then their values are not combined. Use
+   * `Validated` if you prefer to combine `Left` values.
+   *
+   * Examples:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val l1: Either[String, Int] = Either.left("error 1")
+   * scala> val l2: Either[String, Int] = Either.left("error 2")
+   * scala> val r3: Either[String, Int] = Either.right(3)
+   * scala> val r4: Either[String, Int] = Either.right(4)
+   *
+   * scala> l1 combine l2
+   * res0: Either[String, Int] = Left(error 1)
+   *
+   * scala> l1 combine r3
+   * res1: Either[String, Int] = Left(error 1)
+   *
+   * scala> r3 combine l1
+   * res2: Either[String, Int] = Left(error 1)
+   *
+   * scala> r3 combine r4
+   * res3: Either[String, Int] = Right(7)
+   * }}}
+   */
+  final def combine[AA >: A, BB >: B](that: Either[AA, BB])(implicit BB: Semigroup[BB]): Either[AA, BB] = eab match {
+    case left @ Left(_) => left
+    case Right(b1) => that match {
+      case left @ Left(_) => left
+      case Right(b2) => Right(BB.combine(b1, b2))
+    }
+  }
+
+  def show[AA >: A, BB >: B](implicit AA: Show[AA], BB: Show[BB]): String = eab.fold(
+    a => s"Left(${AA.show(a)})",
+    b => s"Right(${BB.show(b)})"
+  )
+
+  def ap[AA >: A, BB >: B, C](that: Either[AA, BB => C]): Either[AA, C] = (new EitherOps(that)).flatMap(this.map)
 
   /**
    * Convert a `scala.util.Either` into a [[cats.data.Xor]].
@@ -27,4 +177,60 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
    * }}}
    */
   def toXor: A Xor B = Xor.fromEither(eab)
+
+  // TODO
+  def toEitherT: Int = ???
+}
+
+final class EitherObjectOps(val either: Either.type) extends AnyVal {
+  def left[A, B](a: A): Either[A, B] = Left(a)
+
+  def right[A, B](b: B): Either[A, B] = Right(b)
+
+  /**
+   * Evaluates the specified block, catching exceptions of the specified type and returning them on the left side of
+   * the resulting `Either`. Uncaught exceptions are propagated.
+   *
+   * For example:
+   * {{{
+   * scala> import cats.implicits._ // get syntax for Either
+   * scala> Either.catchOnly[NumberFormatException] { "foo".toInt }
+   * res0: Either[NumberFormatException, Int] = Left(java.lang.NumberFormatException: For input string: "foo")
+   * }}}
+   */
+  def catchOnly[T >: Null <: Throwable]: CatchOnlyPartiallyApplied[T] =
+    new CatchOnlyPartiallyApplied[T]
+
+  def catchNonFatal[A](f: => A): Either[Throwable, A] =
+    try {
+      right(f)
+    } catch {
+      case scala.util.control.NonFatal(t) => left(t)
+    }
+
+  /**
+   * Converts a `Try[A]` to a `Throwable Xor A`.
+   */
+  def fromTry[A](t: Try[A]): Either[Throwable, A] =
+    t match {
+      case Failure(e) => left(e)
+      case Success(v) => right(v)
+    }
+
+  /**
+   * Converts an `Option[B]` to an `A Xor B`, where the provided `ifNone` values is returned on
+   * the left of the `Xor` when the specified `Option` is `None`.
+   */
+  def fromOption[A, B](o: Option[B], ifNone: => A): Either[A, B] =
+    o.fold(left[A, B](ifNone))(right)
+}
+
+final class CatchOnlyPartiallyApplied[T] private[syntax] {
+  def apply[A](f: => A)(implicit CT: ClassTag[T], NT: NotNull[T]): Either[T, A] =
+    try {
+      Right(f)
+    } catch {
+      case t if CT.runtimeClass.isInstance(t) =>
+        Left(t.asInstanceOf[T])
+    }
 }
diff --git a/core/src/main/scala/cats/syntax/flatMap.scala b/core/src/main/scala/cats/syntax/flatMap.scala
index 057ce263f4..1723dddf83 100644
--- a/core/src/main/scala/cats/syntax/flatMap.scala
+++ b/core/src/main/scala/cats/syntax/flatMap.scala
@@ -25,10 +25,9 @@ final class FlattenOps[F[_], A](ffa: F[F[A]])(implicit F: FlatMap[F]) {
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> type ErrorOr[A] = String Xor A
-   * scala> val x: ErrorOr[ErrorOr[Int]] = Xor.right(Xor.right(3))
+   * scala> type ErrorOr[A] = Either[String, A]
+   * scala> val x: ErrorOr[ErrorOr[Int]] = Right(Right(3))
    * scala> x.flatten
    * res0: ErrorOr[Int] = Right(3)
    * }}}
diff --git a/core/src/main/scala/cats/syntax/monadCombine.scala b/core/src/main/scala/cats/syntax/monadCombine.scala
index 53cbbd4fdb..40e53f2449 100644
--- a/core/src/main/scala/cats/syntax/monadCombine.scala
+++ b/core/src/main/scala/cats/syntax/monadCombine.scala
@@ -33,9 +33,8 @@ final class SeparateOps[F[_], G[_, _], A, B](fgab: F[G[A, B]])(implicit F: Monad
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
-   * scala> val l: List[Xor[String, Int]] = List(Xor.right(1), Xor.left("error"))
+   * scala> val l: List[Either[String, Int]] = List(Right(1), Left("error"))
    * scala> l.separate
    * res0: (List[String], List[Int]) = (List(error),List(1))
    * }}}
diff --git a/docs/src/main/tut/validated.md b/docs/src/main/tut/validated.md
index 216d5f9704..258fd4af4a 100644
--- a/docs/src/main/tut/validated.md
+++ b/docs/src/main/tut/validated.md
@@ -305,19 +305,19 @@ val houseNumber = config.parse[Int]("house_number").andThen{ n =>
 The `withXor` method allows you to temporarily turn a `Validated` instance into an `Xor` instance and apply it to a function.
 
 ```tut:silent
-import cats.data.Xor
+import cats.syntax.either._ // get Either#flatMap
 
-def positive(field: String, i: Int): ConfigError Xor Int = {
-  if (i >= 0) Xor.right(i)
-  else Xor.left(ParseError(field))
+def positive(field: String, i: Int): Either[ConfigError, Int] = {
+  if (i >= 0) Right(i)
+  else Left(ParseError(field))
 }
 ```
 
 Thus.
 
 ```tut:book
-val houseNumber = config.parse[Int]("house_number").withXor{ xor: ConfigError Xor Int =>
-  xor.flatMap{ i =>
+val houseNumber = config.parse[Int]("house_number").withEither{ either: Either[ConfigError, Int] =>
+  either.flatMap{ i =>
     positive("house_number", i)
   }
 }
diff --git a/free/src/main/scala/cats/free/Free.scala b/free/src/main/scala/cats/free/Free.scala
index c027c5b698..7e9e718c05 100644
--- a/free/src/main/scala/cats/free/Free.scala
+++ b/free/src/main/scala/cats/free/Free.scala
@@ -3,7 +3,6 @@ package free
 
 import scala.annotation.tailrec
 
-import cats.data.Xor, Xor.{Left, Right}
 import cats.arrow.FunctionK
 
 /**
@@ -44,7 +43,7 @@ sealed abstract class Free[S[_], A] extends Product with Serializable {
    * Evaluate a single layer of the free monad.
    */
   @tailrec
-  final def resume(implicit S: Functor[S]): S[Free[S, A]] Xor A = this match {
+  final def resume(implicit S: Functor[S]): Either[S[Free[S, A]], A] = this match {
     case Pure(a) => Right(a)
     case Suspend(t) => Left(S.map(t)(Pure(_)))
     case FlatMapped(c, f) =>
@@ -92,20 +91,20 @@ sealed abstract class Free[S[_], A] extends Product with Serializable {
    * resumption in the context of `S`.
    */
   final def runTailRec(implicit S: MonadRec[S]): S[A] = {
-    def step(rma: Free[S, A]): S[Xor[Free[S, A], A]] =
+    def step(rma: Free[S, A]): S[Either[Free[S, A], A]] =
       rma match {
         case Pure(a) =>
-          S.pure(Xor.right(a))
+          S.pure(Right(a))
         case Suspend(ma) =>
-          S.map(ma)(Xor.right(_))
+          S.map(ma)(Right(_))
         case FlatMapped(curr, f) =>
           curr match {
             case Pure(x) =>
-              S.pure(Xor.left(f(x)))
+              S.pure(Left(f(x)))
             case Suspend(mx) =>
-              S.map(mx)(x => Xor.left(f(x)))
+              S.map(mx)(x => Left(f(x)))
             case FlatMapped(prev, g) =>
-              S.pure(Xor.left(prev.flatMap(w => g(w).flatMap(f))))
+              S.pure(Left(prev.flatMap(w => g(w).flatMap(f))))
           }
       }
     S.tailRecM(this)(step)
@@ -121,9 +120,9 @@ sealed abstract class Free[S[_], A] extends Product with Serializable {
    */
   final def foldMap[M[_]](f: FunctionK[S, M])(implicit M: MonadRec[M]): M[A] =
     M.tailRecM(this)(_.step match {
-      case Pure(a) => M.pure(Xor.right(a))
-      case Suspend(sa) => M.map(f(sa))(Xor.right)
-      case FlatMapped(c, g) => M.map(c.foldMap(f))(cc => Xor.left(g(cc)))
+      case Pure(a) => M.pure(Right(a))
+      case Suspend(sa) => M.map(f(sa))(Right(_))
+      case FlatMapped(c, g) => M.map(c.foldMap(f))(cc => Left(g(cc)))
     })
 
   /**
@@ -200,7 +199,7 @@ object Free {
       def pure[A](a: A): Free[S, A] = Free.pure(a)
       override def map[A, B](fa: Free[S, A])(f: A => B): Free[S, B] = fa.map(f)
       def flatMap[A, B](a: Free[S, A])(f: A => Free[S, B]): Free[S, B] = a.flatMap(f)
-      def tailRecM[A, B](a: A)(f: A => Free[S, A Xor B]): Free[S, B] =
+      def tailRecM[A, B](a: A)(f: A => Free[S, Either[A, B]]): Free[S, B] =
         f(a).flatMap(_ match {
           case Left(a1) => tailRecM(a1)(f) // recursion OK here, since Free is lazy
           case Right(b) => pure(b)
diff --git a/free/src/test/scala/cats/free/FreeTests.scala b/free/src/test/scala/cats/free/FreeTests.scala
index d4d6498af2..772b27c67f 100644
--- a/free/src/test/scala/cats/free/FreeTests.scala
+++ b/free/src/test/scala/cats/free/FreeTests.scala
@@ -53,7 +53,7 @@ class FreeTests extends CatsSuite {
   test("tailRecM is stack safe") {
     val n = 50000
     val fa = MonadRec[Free[Option, ?]].tailRecM(0)(i =>
-      Free.pure[Option, Int Xor Int](if (i < n) Xor.Left(i+1) else Xor.Right(i)))
+      Free.pure[Option, Either[Int, Int]](if (i < n) Left(i+1) else Right(i)))
     fa should === (Free.pure[Option, Int](n))
   }
 
diff --git a/free/src/test/scala/cats/free/InjectTests.scala b/free/src/test/scala/cats/free/InjectTests.scala
index ae922e63ba..8e1a97dcaf 100644
--- a/free/src/test/scala/cats/free/InjectTests.scala
+++ b/free/src/test/scala/cats/free/InjectTests.scala
@@ -3,7 +3,7 @@ package free
 
 import cats.arrow.FunctionK
 import cats.tests.CatsSuite
-import cats.data.{Xor, Coproduct}
+import cats.data.Coproduct
 import org.scalacheck._
 
 class InjectTests extends CatsSuite {
@@ -91,13 +91,13 @@ class InjectTests extends CatsSuite {
 
   test("apply in left") {
     forAll { (y: Test1[Int]) =>
-      Inject[Test1Algebra, T].inj(y) == Coproduct(Xor.Left(y)) should ===(true)
+      Inject[Test1Algebra, T].inj(y) == Coproduct(Left(y)) should ===(true)
     }
   }
 
   test("apply in right") {
     forAll { (y: Test2[Int]) =>
-      Inject[Test2Algebra, T].inj(y) == Coproduct(Xor.Right(y)) should ===(true)
+      Inject[Test2Algebra, T].inj(y) == Coproduct(Right(y)) should ===(true)
     }
   }
 
diff --git a/laws/src/main/scala/cats/laws/ApplicativeErrorLaws.scala b/laws/src/main/scala/cats/laws/ApplicativeErrorLaws.scala
index b912534bf7..6dd12d4e21 100644
--- a/laws/src/main/scala/cats/laws/ApplicativeErrorLaws.scala
+++ b/laws/src/main/scala/cats/laws/ApplicativeErrorLaws.scala
@@ -1,7 +1,7 @@
 package cats
 package laws
 
-import cats.data.{Xor, XorT}
+import cats.data.EitherT
 
 // Taken from http://functorial.com/psc-pages/docs/Control/Monad/Error/Class/index.html
 trait ApplicativeErrorLaws[F[_], E] extends ApplicativeLaws[F] {
@@ -19,11 +19,11 @@ trait ApplicativeErrorLaws[F[_], E] extends ApplicativeLaws[F] {
   def handleErrorPure[A](a: A, f: E => A): IsEq[F[A]] =
     F.handleError(F.pure(a))(f) <-> F.pure(a)
 
-  def raiseErrorAttempt(e: E): IsEq[F[E Xor Unit]] =
-    F.attempt(F.raiseError[Unit](e)) <-> F.pure(Xor.left(e))
+  def raiseErrorAttempt(e: E): IsEq[F[Either[E, Unit]]] =
+    F.attempt(F.raiseError[Unit](e)) <-> F.pure(Left(e))
 
-  def pureAttempt[A](a: A): IsEq[F[E Xor A]] =
-    F.attempt(F.pure(a)) <-> F.pure(Xor.right(a))
+  def pureAttempt[A](a: A): IsEq[F[Either[E, A]]] =
+    F.attempt(F.pure(a)) <-> F.pure(Right(a))
 
   def handleErrorWithConsistentWithRecoverWith[A](fa: F[A], f: E => F[A]): IsEq[F[A]] =
     F.handleErrorWith(fa)(f) <-> F.recoverWith(fa)(PartialFunction(f))
@@ -34,8 +34,8 @@ trait ApplicativeErrorLaws[F[_], E] extends ApplicativeLaws[F] {
   def recoverConsistentWithRecoverWith[A](fa: F[A], pf: PartialFunction[E, A]): IsEq[F[A]] =
     F.recover(fa)(pf) <-> F.recoverWith(fa)(pf andThen F.pure)
 
-  def attemptConsistentWithAttemptT[A](fa: F[A]): IsEq[XorT[F, E, A]] =
-    XorT(F.attempt(fa)) <-> F.attemptT(fa)
+  def attemptConsistentWithAttemptT[A](fa: F[A]): IsEq[EitherT[F, E, A]] =
+    EitherT(F.attempt(fa)) <-> F.attemptT(fa)
 }
 
 object ApplicativeErrorLaws {
diff --git a/laws/src/main/scala/cats/laws/ChoiceLaws.scala b/laws/src/main/scala/cats/laws/ChoiceLaws.scala
index 71428186f2..f18f520e20 100644
--- a/laws/src/main/scala/cats/laws/ChoiceLaws.scala
+++ b/laws/src/main/scala/cats/laws/ChoiceLaws.scala
@@ -2,7 +2,6 @@ package cats
 package laws
 
 import cats.arrow.Choice
-import cats.data.Xor
 import cats.syntax.compose._
 
 /**
@@ -11,7 +10,7 @@ import cats.syntax.compose._
 trait ChoiceLaws[F[_, _]] extends CategoryLaws[F] {
   implicit override def F: Choice[F]
 
-  def choiceCompositionDistributivity[A, B, C, D](fac: F[A, C], fbc: F[B, C], fcd: F[C, D]): IsEq[F[Xor[A, B], D]] =
+  def choiceCompositionDistributivity[A, B, C, D](fac: F[A, C], fbc: F[B, C], fcd: F[C, D]): IsEq[F[Either[A, B], D]] =
     (F.choice(fac, fbc) andThen fcd) <-> F.choice(fac andThen fcd, fbc andThen fcd)
 }
 
diff --git a/laws/src/main/scala/cats/laws/FlatMapRecLaws.scala b/laws/src/main/scala/cats/laws/FlatMapRecLaws.scala
index b6eb45aaa9..a95acf7d04 100644
--- a/laws/src/main/scala/cats/laws/FlatMapRecLaws.scala
+++ b/laws/src/main/scala/cats/laws/FlatMapRecLaws.scala
@@ -1,7 +1,6 @@
 package cats
 package laws
 
-import cats.data.Xor
 import cats.syntax.flatMap._
 import cats.syntax.functor._
 
@@ -13,8 +12,8 @@ trait FlatMapRecLaws[F[_]] extends FlatMapLaws[F] {
 
   def tailRecMConsistentFlatMap[A](a: A, f: A => F[A]): IsEq[F[A]] = {
     val bounce = F.tailRecM[(A, Int), A]((a, 1)) { case (a0, i) =>
-      if (i > 0) f(a0).map(a1 => Xor.left((a1, i-1)))
-      else f(a0).map(Xor.right)
+      if (i > 0) f(a0).map(a1 => Left((a1, i-1)))
+      else f(a0).map(Right(_))
     }
     bounce <-> f(a).flatMap(f)
   }
diff --git a/laws/src/main/scala/cats/laws/discipline/ApplicativeErrorTests.scala b/laws/src/main/scala/cats/laws/discipline/ApplicativeErrorTests.scala
index 036e2a8469..8116da9f03 100644
--- a/laws/src/main/scala/cats/laws/discipline/ApplicativeErrorTests.scala
+++ b/laws/src/main/scala/cats/laws/discipline/ApplicativeErrorTests.scala
@@ -2,7 +2,7 @@ package cats
 package laws
 package discipline
 
-import cats.data.{ Xor, XorT }
+import cats.data.EitherT
 import cats.laws.discipline.CartesianTests.Isomorphisms
 import cats.laws.discipline.arbitrary._
 import org.scalacheck.{Arbitrary, Prop}
@@ -22,9 +22,9 @@ trait ApplicativeErrorTests[F[_], E] extends ApplicativeTests[F] {
     EqFB: Eq[F[B]],
     EqFC: Eq[F[C]],
     EqE: Eq[E],
-    EqFXorEU: Eq[F[E Xor Unit]],
-    EqFXorEA: Eq[F[E Xor A]],
-    EqXorTFEA: Eq[XorT[F, E, A]],
+    EqFEitherEU: Eq[F[Either[E, Unit]]],
+    EqFEitherEA: Eq[F[Either[E, A]]],
+    EqEitherTFEA: Eq[EitherT[F, E, A]],
     EqFABC: Eq[F[(A, B, C)]],
     iso: Isomorphisms[F]
   ): RuleSet = {
diff --git a/laws/src/main/scala/cats/laws/discipline/ChoiceTests.scala b/laws/src/main/scala/cats/laws/discipline/ChoiceTests.scala
index 8b2e42f1b1..0244679d67 100644
--- a/laws/src/main/scala/cats/laws/discipline/ChoiceTests.scala
+++ b/laws/src/main/scala/cats/laws/discipline/ChoiceTests.scala
@@ -3,7 +3,6 @@ package laws
 package discipline
 
 import cats.arrow.Choice
-import cats.data.Xor
 import org.scalacheck.Arbitrary
 import org.scalacheck.Prop._
 
@@ -17,7 +16,7 @@ trait ChoiceTests[F[_, _]] extends CategoryTests[F] {
     ArbFCD: Arbitrary[F[C, D]],
     EqFAB: Eq[F[A, B]],
     EqFAD: Eq[F[A, D]],
-    EqFXorABD: Eq[F[Xor[A, B], D]]
+    EqFEitherABD: Eq[F[Either[A, B], D]]
   ): RuleSet =
     new DefaultRuleSet(
       name = "choice",
diff --git a/laws/src/main/scala/cats/laws/discipline/MonadErrorTests.scala b/laws/src/main/scala/cats/laws/discipline/MonadErrorTests.scala
index 732159043b..74bca14988 100644
--- a/laws/src/main/scala/cats/laws/discipline/MonadErrorTests.scala
+++ b/laws/src/main/scala/cats/laws/discipline/MonadErrorTests.scala
@@ -2,7 +2,7 @@ package cats
 package laws
 package discipline
 
-import cats.data.{ Xor, XorT }
+import cats.data.EitherT
 import cats.laws.discipline.CartesianTests.Isomorphisms
 import org.scalacheck.{Arbitrary, Prop}
 import org.scalacheck.Prop.forAll
@@ -21,9 +21,9 @@ trait MonadErrorTests[F[_], E] extends ApplicativeErrorTests[F, E] with MonadTes
     EqFB: Eq[F[B]],
     EqFC: Eq[F[C]],
     EqE: Eq[E],
-    EqFXorEU: Eq[F[E Xor Unit]],
-    EqFXorEA: Eq[F[E Xor A]],
-    EqXorTFEA: Eq[XorT[F, E, A]],
+    EqFEitherEU: Eq[F[Either[E, Unit]]],
+    EqFEitherEA: Eq[F[Either[E, A]]],
+    EqEitherTFEA: Eq[EitherT[F, E, A]],
     EqFABC: Eq[F[(A, B, C)]],
     iso: Isomorphisms[F]
   ): RuleSet = {
diff --git a/tests/src/test/scala/cats/tests/ApplicativeErrorTests.scala b/tests/src/test/scala/cats/tests/ApplicativeErrorTests.scala
index 78b71ac964..7b8d8ab006 100644
--- a/tests/src/test/scala/cats/tests/ApplicativeErrorTests.scala
+++ b/tests/src/test/scala/cats/tests/ApplicativeErrorTests.scala
@@ -1,13 +1,9 @@
 package cats
 package tests
 
-import cats.data.{XorT}
-
+import cats.data.EitherT
 
 class ApplicativeErrorCheck extends CatsSuite {
-
-
-
   val failed: Option[Int] =
     (()).raiseError[Option, Int]
 
@@ -23,12 +19,12 @@ class ApplicativeErrorCheck extends CatsSuite {
     failed.handleErrorWith(_ => Some(7)) should === (Some(7))
   }
 
-  test("attempt syntax creates a wrapped Xor") {
-    failed.attempt should === (Option(().left))
+  test("attempt syntax creates a wrapped Either") {
+    failed.attempt should === (Option(Left(())))
   }
 
-  test("attemptT syntax creates an XorT") {
-    failed.attemptT should === (XorT[Option, Unit, Int](Option(().left)))
+  test("attemptT syntax creates an EitherT") {
+    failed.attemptT should === (EitherT[Option, Unit, Int](Option(Left(()))))
   }
 
   test("recover syntax transforms an error to a success") {
diff --git a/tests/src/test/scala/cats/tests/CoproductTests.scala b/tests/src/test/scala/cats/tests/CoproductTests.scala
index dc2e4c4d55..059c687343 100644
--- a/tests/src/test/scala/cats/tests/CoproductTests.scala
+++ b/tests/src/test/scala/cats/tests/CoproductTests.scala
@@ -53,9 +53,9 @@ class CoproductTests extends CatsSuite {
     }
   }
 
-  test("toValidated + toXor is identity") {
+  test("toValidated + toEither is identity") {
     forAll { (x: Coproduct[Option, List, Int]) =>
-      x.toValidated.toXor should === (x.run)
+      x.toValidated.toEither should === (x.run)
     }
   }
 }
diff --git a/tests/src/test/scala/cats/tests/IorTests.scala b/tests/src/test/scala/cats/tests/IorTests.scala
index eca8a7c9c9..ad79362dc5 100644
--- a/tests/src/test/scala/cats/tests/IorTests.scala
+++ b/tests/src/test/scala/cats/tests/IorTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{Xor, Ior}
+import cats.data.Ior
 import cats.laws.discipline.{BifunctorTests, TraverseTests, MonadTests, SerializableTests, CartesianTests}
 import cats.laws.discipline.arbitrary._
 import org.scalacheck.Arbitrary._
@@ -34,7 +34,7 @@ class IorTests extends CatsSuite {
 
   test("onlyLeftOrRight") {
     forAll { (i: Int Ior String) =>
-      i.onlyLeft.map(Xor.left).orElse(i.onlyRight.map(Xor.right)) should === (i.onlyLeftOrRight)
+      i.onlyLeft.map(Left(_)).orElse(i.onlyRight.map(Right(_))) should === (i.onlyLeftOrRight)
     }
   }
 
@@ -147,9 +147,9 @@ class IorTests extends CatsSuite {
     }
   }
 
-  test("toXor consistent with right") {
+  test("toEither consistent with right") {
     forAll { (x: Int Ior String) =>
-      x.toXor.toOption should === (x.right)
+      x.toEither.toOption should === (x.right)
     }
   }
 
diff --git a/tests/src/test/scala/cats/tests/KleisliTests.scala b/tests/src/test/scala/cats/tests/KleisliTests.scala
index 451a80685c..6153e2bb0c 100644
--- a/tests/src/test/scala/cats/tests/KleisliTests.scala
+++ b/tests/src/test/scala/cats/tests/KleisliTests.scala
@@ -2,7 +2,7 @@ package cats
 package tests
 
 import cats.arrow.{Arrow, Choice, Split, FunctionK}
-import cats.data.{XorT, Kleisli, Reader}
+import cats.data.{EitherT, Kleisli, Reader}
 import cats.functor.{Contravariant, Strong}
 import cats.laws.discipline._
 import cats.laws.discipline.arbitrary._
@@ -15,7 +15,7 @@ class KleisliTests extends CatsSuite {
   implicit def kleisliEq[F[_], A, B](implicit A: Arbitrary[A], FB: Eq[F[B]]): Eq[Kleisli[F, A, B]] =
     Eq.by[Kleisli[F, A, B], A => F[B]](_.run)
 
-  implicit val xorTEq = XorT.catsDataEqForXorT[Kleisli[Option, Int, ?], Unit, Int]
+  implicit val eitherTEq = EitherT.catsDataEqForEitherT[Kleisli[Option, Int, ?], Unit, Int]
 
   implicit val iso = CartesianTests.Isomorphisms.invariant[Kleisli[Option, Int, ?]]
 
@@ -207,4 +207,4 @@ class KleisliTests extends CatsSuite {
     FlatMap[IntReader]
     Semigroup[IntReader[String]]
   }
-}
\ No newline at end of file
+}
diff --git a/tests/src/test/scala/cats/tests/ListWrapper.scala b/tests/src/test/scala/cats/tests/ListWrapper.scala
index 129c587986..6196f26311 100644
--- a/tests/src/test/scala/cats/tests/ListWrapper.scala
+++ b/tests/src/test/scala/cats/tests/ListWrapper.scala
@@ -98,7 +98,7 @@ object ListWrapper {
     new MonadRec[ListWrapper] {
       def pure[A](x: A): ListWrapper[A] = ListWrapper(M.pure(x))
       def flatMap[A, B](fa: ListWrapper[A])(f: A => ListWrapper[B]): ListWrapper[B] = ListWrapper(M.flatMap(fa.list)(a => f(a).list))
-      def tailRecM[A, B](a: A)(f: A => ListWrapper[cats.data.Xor[A,B]]): ListWrapper[B] =
+      def tailRecM[A, B](a: A)(f: A => ListWrapper[Either[A,B]]): ListWrapper[B] =
         ListWrapper(M.tailRecM(a)(a => f(a).list))
     }
   }
diff --git a/tests/src/test/scala/cats/tests/MonadRecInstancesTests.scala b/tests/src/test/scala/cats/tests/MonadRecInstancesTests.scala
index 5839ab419d..9a455e49eb 100644
--- a/tests/src/test/scala/cats/tests/MonadRecInstancesTests.scala
+++ b/tests/src/test/scala/cats/tests/MonadRecInstancesTests.scala
@@ -1,12 +1,12 @@
 package cats
 package tests
 
-import cats.data.{OptionT, StateT, Xor, XorT}
+import cats.data.{EitherT, OptionT, StateT, Xor, XorT}
 
 class MonadRecInstancesTests extends CatsSuite {
   def tailRecMStackSafety[M[_]](implicit M: MonadRec[M], Eq: Eq[M[Int]]): Unit = {
     val n = 50000
-    val res = M.tailRecM(0)(i => M.pure(if (i < n) Xor.Left(i + 1) else Xor.Right(i)))
+    val res = M.tailRecM(0)(i => M.pure(if (i < n) Either.left(i + 1) else Either.right(i)))
     res should === (M.pure(n))
   }
 
@@ -34,6 +34,10 @@ class MonadRecInstancesTests extends CatsSuite {
     tailRecMStackSafety[XorT[Option, String, ?]]
   }
 
+  test("tailRecM stack-safety for EitherT") {
+    tailRecMStackSafety[EitherT[Option, String, ?]]
+  }
+
   test("tailRecM stack-safety for List") {
     tailRecMStackSafety[List]
   }
diff --git a/tests/src/test/scala/cats/tests/OptionTTests.scala b/tests/src/test/scala/cats/tests/OptionTTests.scala
index 6d3dd71fd1..31591b5999 100644
--- a/tests/src/test/scala/cats/tests/OptionTTests.scala
+++ b/tests/src/test/scala/cats/tests/OptionTTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{OptionT, Xor, XorT}
+import cats.data.{EitherT, OptionT}
 import cats.kernel.laws.{GroupLaws, OrderLaws}
 import cats.laws.discipline._
 import cats.laws.discipline.arbitrary._
@@ -82,32 +82,32 @@ class OptionTTests extends CatsSuite {
 
   {
     // F has a MonadError
-    type SXor[A] = String Xor A
+    type SEither[A] = Either[String, A]
 
-    implicit val monadError = OptionT.catsDataMonadErrorForOptionT[SXor, String]
+    implicit val monadError = OptionT.catsDataMonadErrorForOptionT[SEither, String]
 
     import org.scalacheck.Arbitrary
-    implicit val arb1 = implicitly[Arbitrary[OptionT[SXor, Int]]]
-    implicit val arb2 = implicitly[Arbitrary[OptionT[SXor, Int => Int]]]
+    implicit val arb1 = implicitly[Arbitrary[OptionT[SEither, Int]]]
+    implicit val arb2 = implicitly[Arbitrary[OptionT[SEither, Int => Int]]]
 
-    implicit val eq0 = OptionT.catsDataEqForOptionT[SXor, Option[Int]]
-    implicit val eq1 = OptionT.catsDataEqForOptionT[SXor, Int]
-    implicit val eq2 = OptionT.catsDataEqForOptionT[SXor, Unit]
-    implicit val eq3 = OptionT.catsDataEqForOptionT[SXor, SXor[Unit]]
-    implicit val eq4 = OptionT.catsDataEqForOptionT[SXor, SXor[Int]]
-    implicit val eq5 = XorT.catsDataEqForXorT[OptionT[SXor, ?], String, Int]
-    implicit val eq6 = OptionT.catsDataEqForOptionT[SXor, (Int, Int, Int)]
+    implicit val eq0 = OptionT.catsDataEqForOptionT[SEither, Option[Int]]
+    implicit val eq1 = OptionT.catsDataEqForOptionT[SEither, Int]
+    implicit val eq2 = OptionT.catsDataEqForOptionT[SEither, Unit]
+    implicit val eq3 = OptionT.catsDataEqForOptionT[SEither, SEither[Unit]]
+    implicit val eq4 = OptionT.catsDataEqForOptionT[SEither, SEither[Int]]
+    implicit val eq5 = EitherT.catsDataEqForEitherT[OptionT[SEither, ?], String, Int]
+    implicit val eq6 = OptionT.catsDataEqForOptionT[SEither, (Int, Int, Int)]
 
-    implicit val iso = CartesianTests.Isomorphisms.invariant[OptionT[SXor, ?]]
+    implicit val iso = CartesianTests.Isomorphisms.invariant[OptionT[SEither, ?]]
 
-    checkAll("OptionT[String Xor ?, Int]", MonadErrorTests[OptionT[SXor, ?], String].monadError[Int, Int, Int])
-    checkAll("MonadError[OptionT[String Xor ?, ?]]", SerializableTests.serializable(monadError))
+    checkAll("OptionT[Either[String, ?], Int]", MonadErrorTests[OptionT[SEither, ?], String].monadError[Int, Int, Int])
+    checkAll("MonadError[OptionT[Either[String, ?], ?]]", SerializableTests.serializable(monadError))
 
-    Monad[OptionT[SXor, ?]]
-    FlatMap[OptionT[SXor, ?]]
-    Applicative[OptionT[SXor, ?]]
-    Apply[OptionT[SXor, ?]]
-    Functor[OptionT[SXor, ?]]
+    Monad[OptionT[SEither, ?]]
+    FlatMap[OptionT[SEither, ?]]
+    Applicative[OptionT[SEither, ?]]
+    Apply[OptionT[SEither, ?]]
+    Functor[OptionT[SEither, ?]]
   }
 
   {
@@ -233,9 +233,9 @@ class OptionTTests extends CatsSuite {
     }
   }
 
-  test("OptionT[Id, A].toRight consistent with Xor.fromOption") {
+  test("OptionT[Id, A].toRight consistent with Either.fromOption") {
     forAll { (o: OptionT[Id, Int], s: String) =>
-      o.toRight(s).value should === (Xor.fromOption(o.value, s))
+      o.toRight(s).value should === (Either.fromOption(o.value, s))
     }
   }
 
@@ -270,8 +270,8 @@ class OptionTTests extends CatsSuite {
   }
 
   test("show") {
-    val xor: String Xor Option[Int] = Xor.right(Some(1))
-    OptionT[Xor[String, ?], Int](xor).show should === ("Xor.Right(Some(1))")
+    val xor: Either[String, Option[Int]] = Either.right(Some(1))
+    OptionT[Either[String, ?], Int](xor).show should === ("Right(Some(1))")
   }
 
   test("none") {
diff --git a/tests/src/test/scala/cats/tests/ValidatedTests.scala b/tests/src/test/scala/cats/tests/ValidatedTests.scala
index 7ca9b3d55d..646e6f83da 100644
--- a/tests/src/test/scala/cats/tests/ValidatedTests.scala
+++ b/tests/src/test/scala/cats/tests/ValidatedTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{NonEmptyList, Validated, ValidatedNel, Xor, XorT}
+import cats.data.{EitherT, NonEmptyList, Validated, ValidatedNel, Xor}
 import cats.data.Validated.{Valid, Invalid}
 import cats.laws.discipline.{BitraverseTests, TraverseTests, ApplicativeErrorTests, SerializableTests, CartesianTests}
 import org.scalacheck.Arbitrary._
@@ -19,10 +19,10 @@ class ValidatedTests extends CatsSuite {
   checkAll("Validated[?, ?]", BitraverseTests[Validated].bitraverse[Option, Int, Int, Int, String, String, String])
   checkAll("Bitraverse[Validated]", SerializableTests.serializable(Bitraverse[Validated]))
 
-  implicit val eq0 = XorT.catsDataEqForXorT[Validated[String, ?], String, Int]
+  implicit val eq0 = EitherT.catsDataEqForEitherT[Validated[String, ?], String, Int]
 
   checkAll("Validated[String, Int]", ApplicativeErrorTests[Validated[String, ?], String].applicativeError[Int, Int, Int])
-  checkAll("ApplicativeError[Xor, String]", SerializableTests.serializable(ApplicativeError[Validated[String, ?], String]))
+  checkAll("ApplicativeError[Validated, String]", SerializableTests.serializable(ApplicativeError[Validated[String, ?], String]))
 
   checkAll("Validated[String, Int] with Option", TraverseTests[Validated[String,?]].traverse[Int, Int, Int, Int, Option, Option])
   checkAll("Traverse[Validated[String, ?]]", SerializableTests.serializable(Traverse[Validated[String,?]]))
@@ -140,9 +140,9 @@ class ValidatedTests extends CatsSuite {
     }
   }
 
-  test("andThen consistent with Xor's flatMap"){
+  test("andThen consistent with Either's flatMap"){
     forAll { (v: Validated[String, Int], f: Int => Validated[String, Int]) =>
-      v.andThen(f) should === (v.withXor(_.flatMap(f(_).toXor)))
+      v.andThen(f) should === (v.withEither(_.flatMap(f(_).toEither)))
     }
   }
 
diff --git a/tests/src/test/scala/cats/tests/WriterTTests.scala b/tests/src/test/scala/cats/tests/WriterTTests.scala
index ecbc6e9767..c02ad7f7ff 100644
--- a/tests/src/test/scala/cats/tests/WriterTTests.scala
+++ b/tests/src/test/scala/cats/tests/WriterTTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{Validated, Writer, WriterT, XorT}
+import cats.data.{EitherT, Validated, Writer, WriterT}
 import cats.functor.{Bifunctor, Contravariant}
 import cats.laws.discipline._
 import cats.laws.discipline.arbitrary._
@@ -322,8 +322,8 @@ class WriterTTests extends CatsSuite {
     implicit val iso = CartesianTests.Isomorphisms.invariant[WriterT[Validated[String, ?], ListWrapper[Int], ?]]
     implicit def eq1[A:Eq]: Eq[WriterT[Validated[String, ?], ListWrapper[Int], A]] =
       WriterT.catsDataEqForWriterT[Validated[String, ?], ListWrapper[Int], A]
-    implicit val eq2: Eq[XorT[WriterT[Validated[String, ?], ListWrapper[Int], ?], String, Int]] =
-      XorT.catsDataEqForXorT[WriterT[Validated[String, ?], ListWrapper[Int], ?], String, Int]
+    implicit val eq2: Eq[EitherT[WriterT[Validated[String, ?], ListWrapper[Int], ?], String, Int]] =
+      EitherT.catsDataEqForEitherT[WriterT[Validated[String, ?], ListWrapper[Int], ?], String, Int]
     implicit def arb0[A:Arbitrary]: Arbitrary[WriterT[Validated[String, ?], ListWrapper[Int], A]] =
       arbitrary.catsLawsArbitraryForWriterT[Validated[String, ?], ListWrapper[Int], A]
 
@@ -339,7 +339,7 @@ class WriterTTests extends CatsSuite {
     // F has a MonadError and L has a Monoid
     implicit val L: Monoid[ListWrapper[Int]] = ListWrapper.monoid[Int]
     implicit val iso = CartesianTests.Isomorphisms.invariant[WriterT[Option, ListWrapper[Int], ?]]
-    implicit val eq0: Eq[XorT[WriterT[Option, ListWrapper[Int], ?], Unit, Int]] = XorT.catsDataEqForXorT[WriterT[Option, ListWrapper[Int], ?], Unit, Int]
+    implicit val eq0: Eq[EitherT[WriterT[Option, ListWrapper[Int], ?], Unit, Int]] = EitherT.catsDataEqForEitherT[WriterT[Option, ListWrapper[Int], ?], Unit, Int]
 
 
     Functor[WriterT[Option, ListWrapper[Int], ?]]
diff --git a/tests/src/test/scala/cats/tests/XorTTests.scala b/tests/src/test/scala/cats/tests/XorTTests.scala
index 52d86570b7..dfda12bbc6 100644
--- a/tests/src/test/scala/cats/tests/XorTTests.scala
+++ b/tests/src/test/scala/cats/tests/XorTTests.scala
@@ -2,7 +2,7 @@ package cats
 package tests
 
 import cats.functor.Bifunctor
-import cats.data.{Xor, XorT}
+import cats.data.{EitherT, Xor, XorT}
 import cats.laws.discipline._
 import cats.laws.discipline.arbitrary._
 import cats.kernel.laws.{OrderLaws, GroupLaws}
@@ -46,8 +46,8 @@ class XorTTests extends CatsSuite {
   {
     //if a Monad is defined
     implicit val F = ListWrapper.monad
-    implicit val eq0 = XorT.catsDataEqForXorT[ListWrapper, String, String Xor Int]
-    implicit val eq1 = XorT.catsDataEqForXorT[XorT[ListWrapper, String, ?], String, Int](eq0)
+    implicit val eq0 = XorT.catsDataEqForXorT[ListWrapper, String, Either[String, Int]]
+    implicit val eq1 = EitherT.catsDataEqForEitherT[XorT[ListWrapper, String, ?], String, Int](eq0)
 
     Functor[XorT[ListWrapper, String, ?]]
     Applicative[XorT[ListWrapper, String, ?]]
diff --git a/tests/src/test/scala/cats/tests/XorTests.scala b/tests/src/test/scala/cats/tests/XorTests.scala
index 6013e40ee7..ef289473bd 100644
--- a/tests/src/test/scala/cats/tests/XorTests.scala
+++ b/tests/src/test/scala/cats/tests/XorTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{NonEmptyList, Xor, XorT}
+import cats.data.{EitherT, NonEmptyList, Xor}
 import cats.data.Xor._
 import cats.laws.discipline.{SemigroupKTests}
 import cats.laws.discipline.arbitrary._
@@ -22,7 +22,7 @@ class XorTests extends CatsSuite {
 
   checkAll("Xor[String, NonEmptyList[Int]]", GroupLaws[Xor[String, NonEmptyList[Int]]].semigroup)
 
-  implicit val eq0 = XorT.catsDataEqForXorT[Xor[String, ?], String, Int]
+  implicit val eq0 = EitherT.catsDataEqForEitherT[Xor[String, ?], String, Int]
 
   checkAll("Xor[String, Int]", MonadErrorTests[Xor[String, ?], String].monadError[Int, Int, Int])
   checkAll("MonadError[Xor, String]", SerializableTests.serializable(MonadError[Xor[String, ?], String]))

From a43527f3ecbab56ef7555567012a379588f6c39e Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Sun, 14 Aug 2016 20:27:23 -0700
Subject: [PATCH 2/7] Add Either(T) tests and replace Xor with Either in tests

---
 core/src/main/scala/cats/Bitraverse.scala     |  17 +-
 core/src/main/scala/cats/data/EitherT.scala   |   1 -
 .../main/scala/cats/functor/Bifunctor.scala   |   5 +-
 .../main/scala/cats/instances/either.scala    |  14 +
 core/src/main/scala/cats/syntax/either.scala  |  15 +-
 free/src/test/scala/cats/free/FreeTests.scala |  13 +-
 .../test/scala/cats/tests/FutureTests.scala   |   7 +-
 .../test/scala/cats/tests/FutureTests.scala   |   7 +-
 .../cats/laws/discipline/Arbitrary.scala      |   3 +
 .../scala/cats/tests/BifoldableTests.scala    |  12 +-
 .../scala/cats/tests/BitraverseTests.scala    |  12 +-
 .../test/scala/cats/tests/EitherTTests.scala  | 344 ++++++++++++++++++
 .../test/scala/cats/tests/EitherTests.scala   | 191 +++++++++-
 .../scala/cats/tests/MonadCombineTests.scala  |   9 +-
 .../test/scala/cats/tests/OptionTTests.scala  |   4 +-
 .../scala/cats/tests/RegressionTests.scala    |  24 +-
 .../scala/cats/tests/TransLiftTests.scala     |   5 +-
 .../test/scala/cats/tests/UnapplyTests.scala  |   4 +-
 .../scala/cats/tests/ValidatedTests.scala     |   6 +-
 19 files changed, 616 insertions(+), 77 deletions(-)
 create mode 100644 tests/src/test/scala/cats/tests/EitherTTests.scala

diff --git a/core/src/main/scala/cats/Bitraverse.scala b/core/src/main/scala/cats/Bitraverse.scala
index fdb7708082..27767141f4 100644
--- a/core/src/main/scala/cats/Bitraverse.scala
+++ b/core/src/main/scala/cats/Bitraverse.scala
@@ -17,24 +17,23 @@ import simulacrum.typeclass
    *
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
    *
-   * scala> val rightSome: Option[String] Xor Option[Int] = Xor.right(Some(3))
+   * scala> val rightSome: Either[Option[String], Option[Int]] = Either.right(Some(3))
    * scala> rightSome.bisequence
-   * res0: Option[String Xor Int] = Some(Right(3))
+   * res0: Option[Either[String, Int]] = Some(Right(3))
    *
-   * scala> val rightNone: Option[String] Xor Option[Int] = Xor.right(None)
+   * scala> val rightNone: Either[Option[String], Option[Int]] = Either.right(None)
    * scala> rightNone.bisequence
-   * res1: Option[String Xor Int] = None
+   * res1: Option[Either[String, Int]] = None
    *
-   * scala> val leftSome: Option[String] Xor Option[Int] = Xor.left(Some("foo"))
+   * scala> val leftSome: Either[Option[String], Option[Int]] = Either.left(Some("foo"))
    * scala> leftSome.bisequence
-   * res2: Option[String Xor Int] = Some(Left(foo))
+   * res2: Option[Either[String, Int]] = Some(Left(foo))
    *
-   * scala> val leftNone: Option[String] Xor Option[Int] = Xor.left(None)
+   * scala> val leftNone: Either[Option[String], Option[Int]] = Either.left(None)
    * scala> leftNone.bisequence
-   * res3: Option[String Xor Int] = None
+   * res3: Option[Either[String, Int]] = None
    * }}}
    */
   def bisequence[G[_]: Applicative, A, B](fab: F[G[A], G[B]]): G[F[A, B]] =
diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
index c1acc2525e..1e58910f61 100644
--- a/core/src/main/scala/cats/data/EitherT.scala
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -13,7 +13,6 @@ import cats.syntax.either._
  * and lifted in to a `EitherT[F, C, B]` via `EitherT.left`.
  */
 final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
-
   def fold[C](fa: A => C, fb: B => C)(implicit F: Functor[F]): F[C] = F.map(value)(_.fold(fa, fb))
 
   def isLeft(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.isLeft)
diff --git a/core/src/main/scala/cats/functor/Bifunctor.scala b/core/src/main/scala/cats/functor/Bifunctor.scala
index 0d23fc9177..c0140d6f34 100644
--- a/core/src/main/scala/cats/functor/Bifunctor.scala
+++ b/core/src/main/scala/cats/functor/Bifunctor.scala
@@ -41,12 +41,11 @@ import simulacrum.typeclass
    * Widens A into a supertype AA.
    * Example:
    * {{{
-   * scala> import cats.data.Xor
    * scala> import cats.implicits._
    * scala> sealed trait Foo
    * scala> case object Bar extends Foo
-   * scala> val x1: Xor[Bar.type, Int] = Xor.left(Bar)
-   * scala> val x2: Xor[Foo, Int] = x1.leftWiden
+   * scala> val x1: Either[Bar.type, Int] = Either.left(Bar)
+   * scala> val x2: Either[Foo, Int] = x1.leftWiden
    * }}}
    */
   def leftWiden[A, B, AA >: A](fab: F[A, B]): F[AA, B] = fab.asInstanceOf[F[AA, B]]
diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
index 45a4534358..be73acc698 100644
--- a/core/src/main/scala/cats/instances/either.scala
+++ b/core/src/main/scala/cats/instances/either.scala
@@ -95,6 +95,20 @@ trait EitherInstances extends EitherInstances1 {
         b => s"Right(${B.show(b)})"
       )
     }
+
+  implicit def catsDataMonoidForEither[A, B](implicit B: Monoid[B]): Monoid[Either[A, B]] =
+    new Monoid[Either[A, B]] {
+      def empty: Either[A, B] = Right(B.empty)
+      def combine(x: Either[A, B], y: Either[A, B]): Either[A, B] = x combine y
+    }
+
+  implicit def catsDataSemigroupKForEither[L]: SemigroupK[Either[L, ?]] =
+    new SemigroupK[Either[L, ?]] {
+      def combineK[A](x: Either[L, A], y: Either[L, A]): Either[L, A] = x match {
+        case Left(_) => y
+        case Right(_) => x
+      }
+    }
 }
 
 private[instances] sealed trait EitherInstances1 extends EitherInstances2 {
diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index fb728c2860..782db63616 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -1,7 +1,7 @@
 package cats
 package syntax
 
-import cats.data.{Ior, Validated, ValidatedNel, Xor}
+import cats.data.{EitherT, Ior, Validated, ValidatedNel, Xor}
 import scala.reflect.ClassTag
 import scala.util.{Failure, Success, Try}
 
@@ -178,8 +178,17 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
    */
   def toXor: A Xor B = Xor.fromEither(eab)
 
-  // TODO
-  def toEitherT: Int = ???
+  /**
+   * Transform the `Either` into a [[EitherT]] while lifting it into the specified Applicative.
+   *
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val e: Either[String, Int] = Right(3)
+   * scala> e.toEitherT[Option]
+   * res0: cats.data.EitherT[Option, String, Int] = EitherT(Some(Right(3)))
+   * }}}
+   */
+  def toEitherT[F[_]: Applicative]: EitherT[F, A, B] = EitherT.fromEither(eab)
 }
 
 final class EitherObjectOps(val either: Either.type) extends AnyVal {
diff --git a/free/src/test/scala/cats/free/FreeTests.scala b/free/src/test/scala/cats/free/FreeTests.scala
index a219b1a5ed..676a425c5f 100644
--- a/free/src/test/scala/cats/free/FreeTests.scala
+++ b/free/src/test/scala/cats/free/FreeTests.scala
@@ -3,7 +3,6 @@ package free
 
 import cats.tests.CatsSuite
 import cats.arrow.FunctionK
-import cats.data.Xor
 import cats.laws.discipline.{CartesianTests, MonadTests, SerializableTests}
 import cats.laws.discipline.arbitrary.catsLawsArbitraryForFn0
 
@@ -92,18 +91,18 @@ class FreeTests extends CatsSuite {
     // changing the constant argument to .take and observing the time
     // this test takes.
     val ns = Stream.from(1).take(1000)
-    val res = Free.foldLeftM[Stream, Xor[Int, ?], Int, Int](ns, 0) { (sum, n) =>
-      if (sum >= 2) Xor.left(sum) else Xor.right(sum + n)
+    val res = Free.foldLeftM[Stream, Either[Int, ?], Int, Int](ns, 0) { (sum, n) =>
+      if (sum >= 2) Either.left(sum) else Either.right(sum + n)
     }
-    assert(res == Xor.left(3))
+    assert(res == Either.left(3))
   }
 
   test(".foldLeftM short-circuiting") {
     val ns = Stream.continually(1)
-    val res = Free.foldLeftM[Stream, Xor[Int, ?], Int, Int](ns, 0) { (sum, n) =>
-      if (sum >= 100000) Xor.left(sum) else Xor.right(sum + n)
+    val res = Free.foldLeftM[Stream, Either[Int, ?], Int, Int](ns, 0) { (sum, n) =>
+      if (sum >= 100000) Either.left(sum) else Either.right(sum + n)
     }
-    assert(res == Xor.left(100000))
+    assert(res == Either.left(100000))
   }
 }
 
diff --git a/js/src/test/scala/cats/tests/FutureTests.scala b/js/src/test/scala/cats/tests/FutureTests.scala
index 17d69da4fd..ec982d8817 100644
--- a/js/src/test/scala/cats/tests/FutureTests.scala
+++ b/js/src/test/scala/cats/tests/FutureTests.scala
@@ -2,7 +2,6 @@ package cats
 package js
 package tests
 
-import cats.data.Xor
 import cats.laws.discipline._
 import cats.js.instances.Await
 import cats.js.instances.future.futureComonad
@@ -25,13 +24,13 @@ import DeprecatedForwarder.runNow
 class FutureTests extends CatsSuite {
   val timeout = 3.seconds
 
-  def futureXor[A](f: Future[A]): Future[Xor[Throwable, A]] =
-    f.map(Xor.right[Throwable, A]).recover { case t => Xor.left(t) }
+  def futureEither[A](f: Future[A]): Future[Either[Throwable, A]] =
+    f.map(Either.right[Throwable, A]).recover { case t => Either.left(t) }
 
   implicit def eqfa[A: Eq]: Eq[Future[A]] =
     new Eq[Future[A]] {
       def eqv(fx: Future[A], fy: Future[A]): Boolean = {
-        val fz = futureXor(fx) zip futureXor(fy)
+        val fz = futureEither(fx) zip futureEither(fy)
         Await.result(fz.map { case (tx, ty) => tx === ty }, timeout)
       }
     }
diff --git a/jvm/src/test/scala/cats/tests/FutureTests.scala b/jvm/src/test/scala/cats/tests/FutureTests.scala
index 0c283bebb9..af68328617 100644
--- a/jvm/src/test/scala/cats/tests/FutureTests.scala
+++ b/jvm/src/test/scala/cats/tests/FutureTests.scala
@@ -2,7 +2,6 @@ package cats
 package jvm
 package tests
 
-import cats.data.Xor
 import cats.laws.discipline._
 import cats.tests.CatsSuite
 
@@ -16,13 +15,13 @@ import org.scalacheck.Arbitrary.arbitrary
 class FutureTests extends CatsSuite {
   val timeout = 3.seconds
 
-  def futureXor[A](f: Future[A]): Future[Xor[Throwable, A]] =
-    f.map(Xor.right[Throwable, A]).recover { case t => Xor.left(t) }
+  def futureEither[A](f: Future[A]): Future[Either[Throwable, A]] =
+    f.map(Either.right[Throwable, A]).recover { case t => Either.left(t) }
 
   implicit def eqfa[A: Eq]: Eq[Future[A]] =
     new Eq[Future[A]] {
       def eqv(fx: Future[A], fy: Future[A]): Boolean = {
-        val fz = futureXor(fx) zip futureXor(fy)
+        val fz = futureEither(fx) zip futureEither(fy)
         Await.result(fz.map { case (tx, ty) => tx === ty }, timeout)
       }
     }
diff --git a/laws/src/main/scala/cats/laws/discipline/Arbitrary.scala b/laws/src/main/scala/cats/laws/discipline/Arbitrary.scala
index 344ce42f1b..4e9801c6e7 100644
--- a/laws/src/main/scala/cats/laws/discipline/Arbitrary.scala
+++ b/laws/src/main/scala/cats/laws/discipline/Arbitrary.scala
@@ -41,6 +41,9 @@ object arbitrary extends ArbitraryInstances0 {
   implicit def catsLawsArbitraryForXorT[F[_], A, B](implicit F: Arbitrary[F[A Xor B]]): Arbitrary[XorT[F, A, B]] =
     Arbitrary(F.arbitrary.map(XorT(_)))
 
+  implicit def catsLawsArbitraryForEitherT[F[_], A, B](implicit F: Arbitrary[F[Either[A, B]]]): Arbitrary[EitherT[F, A, B]] =
+    Arbitrary(F.arbitrary.map(EitherT(_)))
+
   implicit def catsLawsArbitraryForValidated[A, B](implicit A: Arbitrary[A], B: Arbitrary[B]): Arbitrary[Validated[A, B]] =
     Arbitrary(Gen.oneOf(A.arbitrary.map(Validated.invalid), B.arbitrary.map(Validated.valid)))
 
diff --git a/tests/src/test/scala/cats/tests/BifoldableTests.scala b/tests/src/test/scala/cats/tests/BifoldableTests.scala
index 2c121909bd..8dc75de41c 100644
--- a/tests/src/test/scala/cats/tests/BifoldableTests.scala
+++ b/tests/src/test/scala/cats/tests/BifoldableTests.scala
@@ -1,15 +1,13 @@
 package cats
 package tests
 
-import cats.data.Xor
 import cats.laws.discipline.{BifoldableTests, SerializableTests}
-import cats.laws.discipline.arbitrary._
 
 class BifoldableTest extends CatsSuite {
-  type EitherXor[A, B] = Either[Xor[A, B], Xor[A, B]]
-  val eitherComposeXor: Bifoldable[EitherXor] =
-    Bifoldable[Either].compose[Xor]
+  type EitherEither[A, B] = Either[Either[A, B], Either[A, B]]
+  val eitherComposeEither: Bifoldable[EitherEither] =
+    Bifoldable[Either].compose[Either]
 
-  checkAll("Either compose Xor", BifoldableTests(eitherComposeXor).bifoldable[Int, Int, Int])
-  checkAll("Bifoldable[Either compose Xor]", SerializableTests.serializable(eitherComposeXor))
+  checkAll("Either compose Either", BifoldableTests(eitherComposeEither).bifoldable[Int, Int, Int])
+  checkAll("Bifoldable[Either compose Either]", SerializableTests.serializable(eitherComposeEither))
 }
diff --git a/tests/src/test/scala/cats/tests/BitraverseTests.scala b/tests/src/test/scala/cats/tests/BitraverseTests.scala
index 989c21822a..f438b36b19 100644
--- a/tests/src/test/scala/cats/tests/BitraverseTests.scala
+++ b/tests/src/test/scala/cats/tests/BitraverseTests.scala
@@ -1,15 +1,13 @@
 package cats
 package tests
 
-import cats.data.Xor
 import cats.laws.discipline.{BitraverseTests, SerializableTests}
-import cats.laws.discipline.arbitrary._
 
 class BitraverseTest extends CatsSuite {
-  type XorTuple2[A, B] = Xor[(A, B), (A, B)]
-  val xorComposeTuple2: Bitraverse[XorTuple2] =
-    Bitraverse[Xor].compose[Tuple2]
+  type EitherTuple2[A, B] = Either[(A, B), (A, B)]
+  val eitherComposeTuple2: Bitraverse[EitherTuple2] =
+    Bitraverse[Either].compose[Tuple2]
 
-  checkAll("Xor compose Tuple2", BitraverseTests(xorComposeTuple2).bitraverse[Option, Int, Int, Int, String, String, String])
-  checkAll("Bitraverse[Xor compose Tuple2]", SerializableTests.serializable(xorComposeTuple2))
+  checkAll("Either compose Tuple2", BitraverseTests(eitherComposeTuple2).bitraverse[Option, Int, Int, Int, String, String, String])
+  checkAll("Bitraverse[Either compose Tuple2]", SerializableTests.serializable(eitherComposeTuple2))
 }
diff --git a/tests/src/test/scala/cats/tests/EitherTTests.scala b/tests/src/test/scala/cats/tests/EitherTTests.scala
new file mode 100644
index 0000000000..94ba2b4122
--- /dev/null
+++ b/tests/src/test/scala/cats/tests/EitherTTests.scala
@@ -0,0 +1,344 @@
+package cats
+package tests
+
+import cats.data.EitherT
+import cats.functor.Bifunctor
+import cats.laws.discipline._
+import cats.laws.discipline.arbitrary._
+import cats.kernel.laws.{OrderLaws, GroupLaws}
+
+class EitherTTests extends CatsSuite {
+  implicit val iso = CartesianTests.Isomorphisms.invariant[EitherT[ListWrapper, String, ?]](EitherT.catsDataFunctorForEitherT(ListWrapper.functor))
+
+  {
+    checkAll("EitherT[Option, ListWrapper[String], ?]", SemigroupKTests[EitherT[Option, ListWrapper[String], ?]].semigroupK[Int])
+    checkAll("SemigroupK[EitherT[Option, ListWrapper[String], ?]]", SerializableTests.serializable(SemigroupK[EitherT[Option, ListWrapper[String], ?]]))
+  }
+
+  {
+    implicit val F = ListWrapper.order[Either[String, Int]]
+
+    checkAll("EitherT[List, String, Int]", OrderLaws[EitherT[ListWrapper, String, Int]].order)
+    checkAll("Order[EitherT[List, String, Int]]", SerializableTests.serializable(Order[EitherT[ListWrapper, String, Int]]))
+  }
+
+  {
+    //If a Functor for F is defined
+    implicit val F = ListWrapper.functor
+
+    checkAll("EitherT[ListWrapper, ?, ?]", BifunctorTests[EitherT[ListWrapper, ?, ?]].bifunctor[Int, Int, Int, String, String, String])
+    checkAll("Bifunctor[EitherT[ListWrapper, ?, ?]]", SerializableTests.serializable(Bifunctor[EitherT[ListWrapper, ?, ?]]))
+    checkAll("EitherT[ListWrapper, Int, ?]", FunctorTests[EitherT[ListWrapper, Int, ?]].functor[Int, Int, Int])
+    checkAll("Functor[EitherT[ListWrapper, Int, ?]]", SerializableTests.serializable(Functor[EitherT[ListWrapper, Int, ?]]))
+  }
+
+  {
+    //If a Traverse for F is defined
+    implicit val F = ListWrapper.traverse
+
+    checkAll("EitherT[ListWrapper, Int, ?]", TraverseTests[EitherT[ListWrapper, Int, ?]].traverse[Int, Int, Int, Int, Option, Option])
+    checkAll("Traverse[EitherT[ListWrapper, Int, ?]]", SerializableTests.serializable(Traverse[EitherT[ListWrapper, Int, ?]]))
+    checkAll("EitherT[ListWrapper, ?, ?]", BitraverseTests[EitherT[ListWrapper, ?, ?]].bitraverse[Option, Int, Int, Int, String, String, String])
+    checkAll("Bitraverse[EitherT[ListWrapper, ?, ?]]", SerializableTests.serializable(Bitraverse[EitherT[ListWrapper, ?, ?]]))
+
+  }
+
+  {
+    //if a Monad is defined
+    implicit val F = ListWrapper.monad
+    implicit val eq0 = EitherT.catsDataEqForEitherT[ListWrapper, String, Either[String, Int]]
+    implicit val eq1 = EitherT.catsDataEqForEitherT[EitherT[ListWrapper, String, ?], String, Int](eq0)
+
+    Functor[EitherT[ListWrapper, String, ?]]
+    Applicative[EitherT[ListWrapper, String, ?]]
+    Monad[EitherT[ListWrapper, String, ?]]
+
+    checkAll("EitherT[ListWrapper, String, Int]", MonadErrorTests[EitherT[ListWrapper, String, ?], String].monadError[Int, Int, Int])
+    checkAll("MonadError[EitherT[List, ?, ?]]", SerializableTests.serializable(MonadError[EitherT[ListWrapper, String, ?], String]))
+  }
+
+  {
+    //if a Monad is defined
+    implicit val F = ListWrapper.monad
+
+    Functor[EitherT[ListWrapper, String, ?]]
+    Applicative[EitherT[ListWrapper, String, ?]]
+    Monad[EitherT[ListWrapper, String, ?]]
+
+    checkAll("EitherT[ListWrapper, String, Int]", MonadTests[EitherT[ListWrapper, String, ?]].monad[Int, Int, Int])
+    checkAll("Monad[EitherT[ListWrapper, String, ?]]", SerializableTests.serializable(Monad[EitherT[ListWrapper, String, ?]]))
+  }
+
+  {
+    //If a foldable is defined
+    implicit val F = ListWrapper.foldable
+
+    checkAll("EitherT[ListWrapper, Int, ?]", FoldableTests[EitherT[ListWrapper, Int, ?]].foldable[Int, Int])
+    checkAll("Foldable[EitherT[ListWrapper, Int, ?]]", SerializableTests.serializable(Foldable[EitherT[ListWrapper, Int, ?]]))
+  }
+
+  {
+    implicit val F = ListWrapper.partialOrder[Either[String, Int]]
+
+    checkAll("EitherT[ListWrapper, String, Int]", OrderLaws[EitherT[ListWrapper, String, Int]].partialOrder)
+    checkAll("PartialOrder[EitherT[ListWrapper, String, Int]]", SerializableTests.serializable(PartialOrder[EitherT[ListWrapper, String, Int]]))
+  }
+
+  {
+    implicit val F = ListWrapper.semigroup[Either[String, Int]]
+
+    checkAll("EitherT[ListWrapper, String, Int]", GroupLaws[EitherT[ListWrapper, String, Int]].semigroup)
+    checkAll("Semigroup[EitherT[ListWrapper, String, Int]]", SerializableTests.serializable(Semigroup[EitherT[ListWrapper, String, Int]]))
+  }
+
+  {
+    implicit val F = ListWrapper.monoid[Either[String, Int]]
+
+    Semigroup[EitherT[ListWrapper, String, Int]]
+
+    checkAll("EitherT[ListWrapper, String, Int]", GroupLaws[EitherT[ListWrapper, String, Int]].monoid)
+    checkAll("Monoid[EitherT[ListWrapper, String, Int]]", SerializableTests.serializable(Monoid[EitherT[ListWrapper, String, Int]]))
+  }
+
+  {
+    implicit val F = ListWrapper.eqv[Either[String, Int]]
+
+    checkAll("EitherT[ListWrapper, String, Int]", OrderLaws[EitherT[ListWrapper, String, Int]].eqv)
+    checkAll("Eq[EitherT[ListWrapper, String, Int]]", SerializableTests.serializable(Eq[EitherT[ListWrapper, String, Int]]))
+  }
+
+  test("toValidated") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.toValidated.map(_.toEither) should === (eithert.value)
+    }
+  }
+
+  test("toValidatedNel") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.toValidatedNel.map(_.toEither.leftMap(_.head)) should === (eithert.value)
+    }
+  }
+
+  test("withValidated") {
+    forAll { (eithert: EitherT[List, String, Int], f: String => Char, g: Int => Double) =>
+      eithert.withValidated(_.bimap(f, g)) should === (eithert.bimap(f, g))
+    }
+  }
+
+  test("fromEither") {
+    forAll { (either: Either[String, Int]) =>
+      Some(either.isLeft) should === (EitherT.fromEither[Option](either).isLeft)
+    }
+  }
+
+  test("isLeft negation of isRight") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.isLeft should === (eithert.isRight.map(! _))
+    }
+  }
+
+  test("double swap is noop") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.swap.swap should === (eithert)
+    }
+  }
+
+  test("swap negates isRight") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.swap.isRight should === (eithert.isRight.map(! _))
+    }
+  }
+
+  test("toOption on Right returns Some") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.toOption.isDefined should === (eithert.isRight)
+    }
+  }
+
+  test("toEither preserves isRight") {
+    forAll { (eithert: EitherT[List, String, Int]) =>
+      eithert.value.map(_.isRight) should === (eithert.isRight)
+    }
+  }
+
+  test("recover recovers handled values") {
+    val eithert = EitherT.left[Id, String, Int]("eithert")
+    eithert.recover { case "eithert" => 5 }.isRight should === (true)
+  }
+
+  test("recover ignores unhandled values") {
+    val eithert = EitherT.left[Id, String, Int]("eithert")
+    eithert.recover { case "noteithert" => 5 } should === (eithert)
+  }
+
+  test("recover ignores the right side") {
+    val eithert = EitherT.right[Id, String, Int](10)
+    eithert.recover { case "eithert" => 5 } should === (eithert)
+  }
+
+  test("recoverWith recovers handled values") {
+    val eithert = EitherT.left[Id, String, Int]("eithert")
+    eithert.recoverWith { case "eithert" => EitherT.right[Id, String, Int](5) }.isRight should === (true)
+  }
+
+  test("recoverWith ignores unhandled values") {
+    val eithert = EitherT.left[Id, String, Int]("eithert")
+    eithert.recoverWith { case "noteithert" => EitherT.right[Id, String, Int](5) } should === (eithert)
+  }
+
+  test("recoverWith ignores the right side") {
+    val eithert = EitherT.right[Id, String, Int](10)
+    eithert.recoverWith { case "eithert" => EitherT.right[Id, String, Int](5) } should === (eithert)
+  }
+
+  test("transform consistent with value.map") {
+    forAll { (eithert: EitherT[List, String, Int], f: Either[String, Int] => Either[Long, Double]) =>
+      eithert.transform(f) should === (EitherT(eithert.value.map(f)))
+    }
+  }
+
+  test("semiflatMap consistent with value.flatMap+f+pure") {
+    forAll { (eithert: EitherT[List, String, Int], f: Int => List[String]) =>
+      eithert.semiflatMap(f) should === (EitherT(eithert.value.flatMap {
+        case l @ Left(_) => List(l.asInstanceOf[Either[String, String]])
+        case Right(b) => f(b).map(Right(_))
+      }))
+    }
+  }
+
+  test("subflatMap consistent with value.map+flatMap") {
+    forAll { (eithert: EitherT[List, String, Int], f: Int => Either[String, Double]) =>
+      eithert.subflatMap(f) should === (EitherT(eithert.value.map(_.flatMap(f))))
+    }
+  }
+
+  test("fold with Id consistent with Either fold") {
+    forAll { (eithert: EitherT[Id, String, Int], f: String => Long, g: Int => Long) =>
+      eithert.fold(f, g) should === (eithert.value.fold(f, g))
+    }
+  }
+
+  test("valueOr with Id consistent with Either valueOr") {
+    forAll { (eithert: EitherT[Id, String, Int], f: String => Int) =>
+      eithert.valueOr(f) should === (eithert.value.valueOr(f))
+    }
+  }
+
+  test("getOrElse with Id consistent with Either getOrElse") {
+    forAll { (eithert: EitherT[Id, String, Int], i: Int) =>
+      eithert.getOrElse(i) should === (eithert.value.getOrElse(i))
+    }
+  }
+
+  test("getOrElseF with Id consistent with Either getOrElse") {
+    forAll { (eithert: EitherT[Id, String, Int], i: Int) =>
+      eithert.getOrElseF(i) should === (eithert.value.getOrElse(i))
+    }
+  }
+
+  test("orElse with Id consistent with Either orElse") {
+    forAll { (eithert: EitherT[Id, String, Int], fallback: EitherT[Id, String, Int]) =>
+      eithert.orElse(fallback).value should === (eithert.value.orElse(fallback.value))
+    }
+  }
+
+  test("orElse evaluates effect only once") {
+    forAll { (either: Either[String, Int], fallback: EitherT[Eval, String, Int]) =>
+      var evals = 0
+      val eithert = (EitherT(Eval.always { evals += 1; either }) orElse fallback)
+      eithert.value.value
+      evals should === (1)
+    }
+  }
+
+  test("forall with Id consistent with Either forall") {
+    forAll { (eithert: EitherT[Id, String, Int], f: Int => Boolean) =>
+      eithert.forall(f) should === (eithert.value.forall(f))
+    }
+  }
+
+  test("exists with Id consistent with Either exists") {
+    forAll { (eithert: EitherT[Id, String, Int], f: Int => Boolean) =>
+      eithert.exists(f) should === (eithert.value.exists(f))
+    }
+  }
+
+  test("leftMap with Id consistent with Either leftMap") {
+    forAll { (eithert: EitherT[Id, String, Int], f: String => Long) =>
+      eithert.leftMap(f).value should === (eithert.value.leftMap(f))
+    }
+  }
+
+  test("compare with Id consistent with Either compare") {
+    forAll { (x: EitherT[Id, String, Int], y: EitherT[Id, String, Int]) =>
+      x.compare(y) should === (x.value.compare(y.value))
+    }
+  }
+
+  test("=== with Id consistent with Either ===") {
+    forAll { (x: EitherT[Id, String, Int], y: EitherT[Id, String, Int]) =>
+      x === y should === (x.value === y.value)
+    }
+  }
+
+  test("traverse with Id consistent with Either traverse") {
+    forAll { (x: EitherT[Id, String, Int], f: Int => Option[Long]) =>
+      val e: Either[String, Int] = x.value
+      x.traverse(f).map(_.value) should === (e.traverse(f))
+    }
+  }
+
+  test("foldLeft with Id consistent with Either foldLeft") {
+    forAll { (x: EitherT[Id, String, Int], l: Long, f: (Long, Int) => Long) =>
+      x.foldLeft(l)(f) should === (x.value.foldLeft(l)(f))
+    }
+  }
+
+  test("foldRight with Id consistent with Either foldRight") {
+    forAll { (x: EitherT[Id, String, Int], l: Eval[Long], f: (Int, Eval[Long]) => Eval[Long]) =>
+      x.foldRight(l)(f) should === (x.value.foldRight(l)(f))
+    }
+  }
+
+  test("merge with Id consistent with Either merge") {
+    forAll { (x: EitherT[Id, Int, Int]) =>
+      x.merge should === (x.value.merge)
+    }
+  }
+
+  test("to consistent with toOption") {
+    forAll { (x: EitherT[List, String, Int]) =>
+      x.to[Option] should === (x.toOption.value)
+    }
+  }
+
+  test("toEither consistent with toOption") {
+    forAll { (x: EitherT[List, String, Int]) =>
+      x.value.map(_.right.toOption) should === (x.toOption.value)
+    }
+  }
+
+  test("ensure on left is identity") {
+    forAll { (x: EitherT[Id, String, Int], s: String, p: Int => Boolean) =>
+      if (x.isLeft) {
+        x.ensure(s)(p) should === (x)
+      }
+    }
+  }
+
+  test("ensure on right is identity if predicate satisfied") {
+    forAll { (x: EitherT[Id, String, Int], s: String, p: Int => Boolean) =>
+      if (x.isRight && p(x getOrElse 0)) {
+        x.ensure(s)(p) should === (x)
+      }
+    }
+  }
+
+  test("ensure should fail if predicate not satisfied") {
+    forAll { (x: EitherT[Id, String, Int], s: String, p: Int => Boolean) =>
+      if (x.isRight && !p(x getOrElse 0)) {
+        x.ensure(s)(p) should === (EitherT.left[Id, String, Int](s))
+      }
+    }
+  }
+}
diff --git a/tests/src/test/scala/cats/tests/EitherTests.scala b/tests/src/test/scala/cats/tests/EitherTests.scala
index 97d7d74e08..8e4641e911 100644
--- a/tests/src/test/scala/cats/tests/EitherTests.scala
+++ b/tests/src/test/scala/cats/tests/EitherTests.scala
@@ -1,18 +1,23 @@
 package cats
 package tests
 
-import cats.laws.discipline.{BitraverseTests, TraverseTests, MonadTests, SerializableTests, CartesianTests}
-import cats.kernel.laws.OrderLaws
+import cats.data.EitherT
+import cats.laws.discipline._
+import cats.kernel.laws.{GroupLaws, OrderLaws}
+import scala.util.Try
 
 class EitherTests extends CatsSuite {
-
   implicit val iso = CartesianTests.Isomorphisms.invariant[Either[Int, ?]]
 
+  checkAll("Either[String, Int]", GroupLaws[Either[String, Int]].monoid)
+
   checkAll("Either[Int, Int]", CartesianTests[Either[Int, ?]].cartesian[Int, Int, Int])
   checkAll("Cartesian[Either[Int, ?]]", SerializableTests.serializable(Cartesian[Either[Int, ?]]))
 
-  checkAll("Either[Int, Int]", MonadTests[Either[Int, ?]].monad[Int, Int, Int])
-  checkAll("Monad[Either[Int, ?]]", SerializableTests.serializable(Monad[Either[Int, ?]]))
+  implicit val eq0 = EitherT.catsDataEqForEitherT[Either[Int, ?], Int, Int]
+
+  checkAll("Either[Int, Int]", MonadErrorTests[Either[Int, ?], Int].monadError[Int, Int, Int])
+  checkAll("MonadError[Either[Int, ?]]", SerializableTests.serializable(MonadError[Either[Int, ?], Int]))
 
   checkAll("Either[Int, Int] with Option", TraverseTests[Either[Int, ?]].traverse[Int, Int, Int, Int, Option, Option])
   checkAll("Traverse[Either[Int, ?]", SerializableTests.serializable(Traverse[Either[Int, ?]]))
@@ -20,6 +25,9 @@ class EitherTests extends CatsSuite {
   checkAll("Either[?, ?]", BitraverseTests[Either].bitraverse[Option, Int, Int, Int, String, String, String])
   checkAll("Bitraverse[Either]", SerializableTests.serializable(Bitraverse[Either]))
 
+  checkAll("Either[ListWrapper[String], ?]", SemigroupKTests[Either[ListWrapper[String], ?]].semigroupK[Int])
+  checkAll("SemigroupK[Either[ListWrapper[String], ?]]", SerializableTests.serializable(SemigroupK[Either[ListWrapper[String], ?]]))
+
   val partialOrder = catsStdPartialOrderForEither[Int, String]
   val order = implicitly[Order[Either[Int, String]]]
   val monad = implicitly[Monad[Either[Int, ?]]]
@@ -55,4 +63,177 @@ class EitherTests extends CatsSuite {
     val x: Either[String, Int] = Left("l")
     x.map2Eval(bomb)(_ + _).value should === (x)
   }
+
+  test("catchOnly lets non-matching exceptions escape") {
+    val _ = intercept[NumberFormatException] {
+      Either.catchOnly[IndexOutOfBoundsException]{ "foo".toInt }
+    }
+  }
+
+  test("catchNonFatal catches non-fatal exceptions") {
+    assert(Either.catchNonFatal{ "foo".toInt }.isLeft)
+    assert(Either.catchNonFatal{ throw new Throwable("blargh") }.isLeft)
+  }
+
+  test("fromTry is left for failed Try") {
+    forAll { t: Try[Int] =>
+      t.isFailure should === (Either.fromTry(t).isLeft)
+    }
+  }
+
+  test("fromOption isLeft consistent with Option.isEmpty") {
+    forAll { (o: Option[Int], s: String) =>
+      Either.fromOption(o, s).isLeft should === (o.isEmpty)
+    }
+  }
+
+  test("double swap is identity") {
+    forAll { (x: Either[Int, String]) =>
+      x.swap.swap should === (x)
+    }
+  }
+
+  test("swap negates isLeft/isRight") {
+    forAll { (x: Either[Int, String]) =>
+      x.isLeft should !== (x.swap.isLeft)
+      x.isRight should !== (x.swap.isRight)
+    }
+  }
+
+  test("isLeft consistent with isRight") {
+    forAll { (x: Either[Int, String]) =>
+      x.isLeft should !== (x.isRight)
+    }
+  }
+
+  test("foreach is noop for left") {
+    forAll { (x: Either[Int, String]) =>
+      var count = 0
+      x.foreach{ _ => count += 1}
+      (count == 0) should === (x.isLeft)
+    }
+  }
+
+  test("getOrElse ignores default for right") {
+    forAll { (x: Either[Int, String], s: String, t: String) =>
+      if (x.isRight) {
+        x.getOrElse(s) should === (x.getOrElse(t))
+      }
+    }
+  }
+
+  test("orElse") {
+    forAll { (x: Either[Int, String], y: Either[Int, String]) =>
+      val z = x.orElse(y)
+      (z === (x)) || (z === (y)) should === (true)
+    }
+  }
+
+  test("recover recovers handled values") {
+    val either = Either.left[String, Int]("either")
+    either.recover { case "either" => 5 }.isRight should === (true)
+  }
+
+  test("recover ignores unhandled values") {
+    val either = Either.left[String, Int]("either")
+    either.recover { case "noteither" => 5 } should === (either)
+  }
+
+  test("recover ignores the right side") {
+    val either = Either.right[String, Int](10)
+    either.recover { case "either" => 5 } should === (either)
+  }
+
+  test("recoverWith recovers handled values") {
+    val either = Either.left[String, Int]("either")
+    either.recoverWith { case "either" => Either.right[String, Int](5) }.isRight should === (true)
+  }
+
+  test("recoverWith ignores unhandled values") {
+    val either = Either.left[String, Int]("either")
+    either.recoverWith { case "noteither" => Either.right[String, Int](5) } should === (either)
+  }
+
+  test("recoverWith ignores the right side") {
+    val either = Either.right[String, Int](10)
+    either.recoverWith { case "either" => Either.right[String, Int](5) } should === (either)
+  }
+
+  test("valueOr consistent with swap then map then merge") {
+    forAll { (x: Either[Int, String], f: Int => String) =>
+      x.valueOr(f) should === (x.swap.map(f).merge)
+    }
+  }
+
+  test("isLeft implies forall") {
+    forAll { (x: Either[Int, String], p: String => Boolean) =>
+      if (x.isLeft) {
+        x.forall(p) should === (true)
+      }
+    }
+  }
+
+  test("isLeft implies exists is false") {
+    forAll { (x: Either[Int, String], p: String => Boolean) =>
+      if (x.isLeft) {
+        x.exists(p) should === (false)
+      }
+    }
+  }
+
+  test("ensure on left is identity") {
+    forAll { (x: Either[Int, String], i: Int, p: String => Boolean) =>
+      if (x.isLeft) {
+        x.ensure(i)(p) should === (x)
+      }
+    }
+  }
+
+  test("toIor then toEither is identity") {
+    forAll { (x: Either[Int, String]) =>
+      x.toIor.toEither should === (x)
+    }
+  }
+
+  test("toTry then fromTry is identity") {
+    implicit def eqTh: Eq[Throwable] = Eq.allEqual
+
+    forAll { (x: Throwable Either String) =>
+      Either.fromTry(x.toTry) should === (x)
+    }
+  }
+
+  test("isLeft consistency") {
+    forAll { (x: Either[Int, String]) =>
+      x.isLeft should === (x.toOption.isEmpty)
+      x.isLeft should === (x.toList.isEmpty)
+      x.isLeft should === (x.toValidated.isInvalid)
+      x.isLeft should === (x.toValidatedNel.isInvalid)
+      Option(x.isLeft) should === (x.toEitherT[Option].isLeft)
+    }
+  }
+
+  test("withValidated") {
+    forAll { (x: Either[Int, String], f: Int => Double) =>
+      x.withValidated(_.bimap(f, identity)) should === (x.leftMap(f))
+    }
+  }
+
+  test("combine is right iff both operands are right") {
+    forAll { (x: Either[Int, String], y: Either[Int, String]) =>
+      x.combine(y).isRight should === (x.isRight && y.isRight)
+    }
+  }
+
+  test("to consistent with toList") {
+    forAll { (x: Either[Int, String]) =>
+      x.to[List, String] should === (x.toList)
+    }
+  }
+
+  test("to consistent with toOption") {
+    forAll { (x: Either[Int, String]) =>
+      x.to[Option, String] should === (x.toOption)
+    }
+  }
 }
diff --git a/tests/src/test/scala/cats/tests/MonadCombineTests.scala b/tests/src/test/scala/cats/tests/MonadCombineTests.scala
index 6a4901b10a..c9f11e234e 100644
--- a/tests/src/test/scala/cats/tests/MonadCombineTests.scala
+++ b/tests/src/test/scala/cats/tests/MonadCombineTests.scala
@@ -1,14 +1,11 @@
 package cats
 package tests
 
-import cats.data.Xor
-import cats.laws.discipline.arbitrary.catsLawsArbitraryForXor
-
 class MonadCombineTest extends CatsSuite {
   test("separate") {
-    forAll { (list: List[Xor[Int, String]]) =>
-      val ints = list.collect { case Xor.Left(i) => i }
-      val strings = list.collect { case Xor.Right(s) => s }
+    forAll { (list: List[Either[Int, String]]) =>
+      val ints = list.collect { case Left(i) => i }
+      val strings = list.collect { case Right(s) => s }
       val expected = (ints, strings)
 
       MonadCombine[List].separate(list) should === (expected)
diff --git a/tests/src/test/scala/cats/tests/OptionTTests.scala b/tests/src/test/scala/cats/tests/OptionTTests.scala
index 0b2a82789a..041d72d0e0 100644
--- a/tests/src/test/scala/cats/tests/OptionTTests.scala
+++ b/tests/src/test/scala/cats/tests/OptionTTests.scala
@@ -256,8 +256,8 @@ class OptionTTests extends CatsSuite {
   }
 
   test("show") {
-    val xor: Either[String, Option[Int]] = Either.right(Some(1))
-    OptionT[Either[String, ?], Int](xor).show should === ("Right(Some(1))")
+    val either: Either[String, Option[Int]] = Either.right(Some(1))
+    OptionT[Either[String, ?], Int](either).show should === ("Right(Some(1))")
   }
 
   test("none") {
diff --git a/tests/src/test/scala/cats/tests/RegressionTests.scala b/tests/src/test/scala/cats/tests/RegressionTests.scala
index 3eed4329ec..50f000c87a 100644
--- a/tests/src/test/scala/cats/tests/RegressionTests.scala
+++ b/tests/src/test/scala/cats/tests/RegressionTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{Const, NonEmptyList, Xor}
+import cats.data.{Const, NonEmptyList}
 
 import scala.collection.mutable
 
@@ -89,11 +89,11 @@ class RegressionTests extends CatsSuite {
     )
   }
 
-  test("#513: traverse short circuits - Xor") {
+  test("#513: traverse short circuits - Either") {
     var count = 0
-    def validate(i: Int): Xor[String, Int] = {
+    def validate(i: Int): Either[String, Int] = {
       count = count + 1
-      if (i < 5) Xor.right(i) else Xor.left(s"$i is greater than 5")
+      if (i < 5) Either.right(i) else Either.left(s"$i is greater than 5")
     }
 
     def checkAndResetCount(expected: Int): Unit = {
@@ -101,31 +101,31 @@ class RegressionTests extends CatsSuite {
       count = 0
     }
 
-    List(1,2,6,8).traverseU(validate) should === (Xor.left("6 is greater than 5"))
+    List(1,2,6,8).traverseU(validate) should === (Either.left("6 is greater than 5"))
     // shouldn't have ever evaluted validate(8)
     checkAndResetCount(3)
 
-    Stream(1,2,6,8).traverseU(validate) should === (Xor.left("6 is greater than 5"))
+    Stream(1,2,6,8).traverseU(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(3)
 
     type StringMap[A] = Map[String, A]
     val intMap: StringMap[Int] = Map("one" -> 1, "two" -> 2, "six" -> 6, "eight" -> 8)
-    intMap.traverseU(validate) should === (Xor.left("6 is greater than 5"))
+    intMap.traverseU(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(3)
 
-    NonEmptyList.of(1,2,6,8).traverseU(validate) should === (Xor.left("6 is greater than 5"))
+    NonEmptyList.of(1,2,6,8).traverseU(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(3)
 
-    NonEmptyList.of(6,8).traverseU(validate) should === (Xor.left("6 is greater than 5"))
+    NonEmptyList.of(6,8).traverseU(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(1)
 
-    List(1,2,6,8).traverseU_(validate) should === (Xor.left("6 is greater than 5"))
+    List(1,2,6,8).traverseU_(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(3)
 
-    NonEmptyList.of(1,2,6,7,8).traverseU_(validate) should === (Xor.left("6 is greater than 5"))
+    NonEmptyList.of(1,2,6,7,8).traverseU_(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(3)
 
-    NonEmptyList.of(6,7,8).traverseU_(validate) should === (Xor.left("6 is greater than 5"))
+    NonEmptyList.of(6,7,8).traverseU_(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(1)
   }
 }
diff --git a/tests/src/test/scala/cats/tests/TransLiftTests.scala b/tests/src/test/scala/cats/tests/TransLiftTests.scala
index fbc354b7d7..befa32177c 100644
--- a/tests/src/test/scala/cats/tests/TransLiftTests.scala
+++ b/tests/src/test/scala/cats/tests/TransLiftTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import data.{OptionT,XorT,WriterT,Kleisli, StateT}
+import cats.data.{EitherT,OptionT,XorT,WriterT,Kleisli, StateT}
 
 class TransLiftTests extends CatsSuite {
 
@@ -22,7 +22,8 @@ class TransLiftTests extends CatsSuite {
     override def map[A, B](fa: JustAp[A])(f: A => B): JustAp[B] = JustAp(f(fa.a))
   }
 
-  test("transLift for XorT, OptionT, WriterT requires only Functor") {
+  test("transLift for EitherT, XorT, OptionT, WriterT requires only Functor") {
+    val e: EitherT[JustFunctor, Int, Int]    = JustFunctor(1).liftT[λ[(α[_], β) => EitherT[α, Int, β]]]
     val d: XorT[JustFunctor, Int, Int]    = JustFunctor(1).liftT[λ[(α[_], β) => XorT[α, Int, β]]]
     val c: OptionT[JustFunctor, Int]      = JustFunctor(1).liftT[OptionT]
     val a: WriterT[JustFunctor, Int, Int] = JustFunctor(1).liftT[λ[(α[_], β) => WriterT[α, Int, β]]]
diff --git a/tests/src/test/scala/cats/tests/UnapplyTests.scala b/tests/src/test/scala/cats/tests/UnapplyTests.scala
index 6eccf76e18..623dbd3381 100644
--- a/tests/src/test/scala/cats/tests/UnapplyTests.scala
+++ b/tests/src/test/scala/cats/tests/UnapplyTests.scala
@@ -14,8 +14,8 @@ class UnapplyTests extends CatsSuite {
   }
 
   test("Unapply works for F[_,_] with the left fixed") {
-    val x = Traverse[List].traverseU(List(1,2,3))(Xor.right(_))
-    (x: String Xor List[Int]) should === (Xor.right(List(1,2,3)))
+    val x = Traverse[List].traverseU(List(1,2,3))(Either.right(_))
+    (x: Either[String, List[Int]]) should === (Either.right(List(1,2,3)))
   }
 
   test("Unapply works for F[_[_],_] with the left fixed") {
diff --git a/tests/src/test/scala/cats/tests/ValidatedTests.scala b/tests/src/test/scala/cats/tests/ValidatedTests.scala
index 646e6f83da..d40ee2cf6e 100644
--- a/tests/src/test/scala/cats/tests/ValidatedTests.scala
+++ b/tests/src/test/scala/cats/tests/ValidatedTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{EitherT, NonEmptyList, Validated, ValidatedNel, Xor}
+import cats.data.{EitherT, NonEmptyList, Validated, ValidatedNel}
 import cats.data.Validated.{Valid, Invalid}
 import cats.laws.discipline.{BitraverseTests, TraverseTests, ApplicativeErrorTests, SerializableTests, CartesianTests}
 import org.scalacheck.Arbitrary._
@@ -156,9 +156,9 @@ class ValidatedTests extends CatsSuite {
     (Validated.invalid("foo") andThen even) should === (Validated.invalid("foo"))
   }
 
-  test("fromOption consistent with Xor.fromOption"){
+  test("fromOption consistent with Either.fromOption"){
     forAll { (o: Option[Int], s: String) =>
-      Validated.fromOption(o, s) should === (Xor.fromOption(o, s).toValidated)
+      Validated.fromOption(o, s) should === (Either.fromOption(o, s).toValidated)
     }
   }
 

From 8fc564e25067e45f8255cf54854f5f356a462149 Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Sun, 14 Aug 2016 20:50:45 -0700
Subject: [PATCH 3/7] ScalaStyle fixes

---
 core/src/main/scala/cats/instances/either.scala | 1 +
 core/src/main/scala/cats/syntax/either.scala    | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
index be73acc698..2058904920 100644
--- a/core/src/main/scala/cats/instances/either.scala
+++ b/core/src/main/scala/cats/instances/either.scala
@@ -26,6 +26,7 @@ trait EitherInstances extends EitherInstances1 {
         }
     }
 
+  // scalastyle:off method.length
   implicit def catsStdInstancesForEither[A]: MonadError[Either[A, ?], A] with Traverse[Either[A, ?]] with RecursiveTailRecM[Either[A, ?]] =
     new MonadError[Either[A, ?], A] with Traverse[Either[A, ?]] with RecursiveTailRecM[Either[A, ?]] {
       def pure[B](b: B): Either[A, B] = Right(b)
diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index 782db63616..2d44f219b9 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -8,7 +8,7 @@ import scala.util.{Failure, Success, Try}
 trait EitherSyntax {
   implicit def catsSyntaxEither[A, B](eab: Either[A, B]): EitherOps[A, B] = new EitherOps(eab)
 
-  implicit def catsSyntaxEitherObject(either: Either.type): EitherObjectOps = new EitherObjectOps(either)
+  implicit def catsSyntaxEitherObject(either: Either.type ): EitherObjectOps = new EitherObjectOps(either)
 }
 
 final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
@@ -191,7 +191,7 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   def toEitherT[F[_]: Applicative]: EitherT[F, A, B] = EitherT.fromEither(eab)
 }
 
-final class EitherObjectOps(val either: Either.type) extends AnyVal {
+final class EitherObjectOps(val either: Either.type ) extends AnyVal {
   def left[A, B](a: A): Either[A, B] = Left(a)
 
   def right[A, B](b: B): Either[A, B] = Right(b)

From 4530d3aa93131ec28096968a3c903ed1016dbf1b Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Mon, 15 Aug 2016 13:29:51 -0700
Subject: [PATCH 4/7] Fix ScalaDoc, address comments

- Remove variance workarounds since Either syntax is invariant
- Prefer pattern matching over fold for performance
- Fix ScalaDoc
- Replace ad-hoc casts with leftCast and rightCast
- Re-enable ScalaStyle after disables and disable ScalaStyle on Either.type enrichment
---
 .../main/scala/cats/ApplicativeError.scala    |   2 +-
 core/src/main/scala/cats/FlatMap.scala        |   2 +-
 core/src/main/scala/cats/data/EitherT.scala   |  28 +--
 core/src/main/scala/cats/data/Ior.scala       |   2 +-
 .../main/scala/cats/instances/either.scala    |   1 +
 core/src/main/scala/cats/syntax/either.scala  | 195 ++++++++++++------
 .../test/scala/cats/tests/EitherTests.scala   |   4 +-
 7 files changed, 153 insertions(+), 81 deletions(-)

diff --git a/core/src/main/scala/cats/ApplicativeError.scala b/core/src/main/scala/cats/ApplicativeError.scala
index ee1e473539..4f36625205 100644
--- a/core/src/main/scala/cats/ApplicativeError.scala
+++ b/core/src/main/scala/cats/ApplicativeError.scala
@@ -39,7 +39,7 @@ trait ApplicativeError[F[_], E] extends Applicative[F] {
   /**
    * Handle errors by turning them into [[scala.util.Either]] values.
    *
-   * If there is no error, then an [[scala.util.Right]] value will be returned instead.
+   * If there is no error, then an `scala.util.Right` value will be returned instead.
    *
    * All non-fatal errors should be handled by this method.
    */
diff --git a/core/src/main/scala/cats/FlatMap.scala b/core/src/main/scala/cats/FlatMap.scala
index 979dda9bac..a7767ee808 100644
--- a/core/src/main/scala/cats/FlatMap.scala
+++ b/core/src/main/scala/cats/FlatMap.scala
@@ -92,7 +92,7 @@ import simulacrum.typeclass
     flatMap(fa)(if (_) ifTrue else ifFalse)
 
   /**
-   * Keeps calling `f` until a `[[Right]][B]` is returned.
+   * Keeps calling `f` until a `scala.util.Right[B]` is returned.
    *
    * Based on Phil Freeman's
    * [[http://functorial.com/stack-safety-for-free/index.pdf Stack Safety for Free]].
diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
index 1e58910f61..c9649e926f 100644
--- a/core/src/main/scala/cats/data/EitherT.scala
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -21,19 +21,19 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
 
   def swap(implicit F: Functor[F]): EitherT[F, B, A] = EitherT(F.map(value)(_.swap))
 
-  def getOrElse[BB >: B](default: => BB)(implicit F: Functor[F]): F[BB] = F.map(value)(_.getOrElse(default))
+  def getOrElse(default: => B)(implicit F: Functor[F]): F[B] = F.map(value)(_.getOrElse(default))
 
-  def getOrElseF[BB >: B](default: => F[BB])(implicit F: Monad[F]): F[BB] = {
+  def getOrElseF(default: => F[B])(implicit F: Monad[F]): F[B] = {
     F.flatMap(value) {
       case Left(_) => default
       case Right(b) => F.pure(b)
     }
   }
 
-  def orElse[AA, BB >: B](default: => EitherT[F, AA, BB])(implicit F: Monad[F]): EitherT[F, AA, BB] = {
+  def orElse(default: => EitherT[F, A, B])(implicit F: Monad[F]): EitherT[F, A, B] = {
     EitherT(F.flatMap(value) {
       case Left(_) => default.value
-      case r @ Right(_) => F.pure(r.asInstanceOf[Either[AA, BB]])
+      case r @ Right(_) => F.pure(r)
     })
   }
 
@@ -46,21 +46,21 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
       case other => F.pure(other)
     })
 
-  def valueOr[BB >: B](f: A => BB)(implicit F: Functor[F]): F[BB] = fold(f, identity)
+  def valueOr(f: A => B)(implicit F: Functor[F]): F[B] = fold(f, identity)
 
   def forall(f: B => Boolean)(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.forall(f))
 
   def exists(f: B => Boolean)(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.exists(f))
 
-  def ensure[AA >: A](onFailure: => AA)(f: B => Boolean)(implicit F: Functor[F]): EitherT[F, AA, B] = EitherT(F.map(value)(_.ensure(onFailure)(f)))
+  def ensure(onFailure: => A)(f: B => Boolean)(implicit F: Functor[F]): EitherT[F, A, B] = EitherT(F.map(value)(_.ensure(onFailure)(f)))
 
   def toOption(implicit F: Functor[F]): OptionT[F, B] = OptionT(F.map(value)(_.toOption))
 
   def to[G[_]](implicit F: Functor[F], G: Alternative[G]): F[G[B]] =
-    F.map(value)(_.to[G, B])
+    F.map(value)(_.to[G])
 
   def collectRight(implicit F: MonadCombine[F]): F[B] =
-    F.flatMap(value)(_.to[F, B])
+    F.flatMap(value)(_.to[F])
 
   def bimap[C, D](fa: A => C, fb: B => D)(implicit F: Functor[F]): EitherT[F, C, D] = EitherT(F.map(value)(_.bimap(fa, fb)))
 
@@ -70,19 +70,19 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
   def applyAlt[D](ff: EitherT[F, A, B => D])(implicit F: Apply[F]): EitherT[F, A, D] =
     EitherT[F, A, D](F.map2(this.value, ff.value)((xb, xbd) => Apply[Either[A, ?]].ap(xbd)(xb)))
 
-  def flatMap[AA >: A, D](f: B => EitherT[F, AA, D])(implicit F: Monad[F]): EitherT[F, AA, D] =
+  def flatMap[D](f: B => EitherT[F, A, D])(implicit F: Monad[F]): EitherT[F, A, D] =
     EitherT(F.flatMap(value) {
-      case l @ Left(_) => F.pure(l.asInstanceOf[Either[AA, D]])
+      case l @ Left(_) => F.pure((l: Either[A, B]).rightCast[D])
       case Right(b) => f(b).value
     })
 
-  def flatMapF[AA >: A, D](f: B => F[Either[AA, D]])(implicit F: Monad[F]): EitherT[F, AA, D] =
+  def flatMapF[D](f: B => F[Either[A, D]])(implicit F: Monad[F]): EitherT[F, A, D] =
     flatMap(f andThen EitherT.apply)
 
   def transform[C, D](f: Either[A, B] => Either[C, D])(implicit F: Functor[F]): EitherT[F, C, D] =
     EitherT(F.map(value)(f))
 
-  def subflatMap[AA >: A, D](f: B => Either[AA, D])(implicit F: Functor[F]): EitherT[F, AA, D] =
+  def subflatMap[D](f: B => Either[A, D])(implicit F: Functor[F]): EitherT[F, A, D] =
     transform(_.flatMap(f))
 
   def map[D](f: B => D)(implicit F: Functor[F]): EitherT[F, A, D] = bimap(identity, f)
@@ -110,10 +110,10 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
   def foldRight[C](lc: Eval[C])(f: (B, Eval[C]) => Eval[C])(implicit F: Foldable[F]): Eval[C] =
     F.foldRight(value, lc)((axb, lc) => axb.foldRight(lc)(f))
 
-  def merge[AA >: A](implicit ev: B <:< AA, F: Functor[F]): F[AA] = F.map(value)(_.fold(identity, ev.apply))
+  def merge(implicit ev: B <:< A, F: Functor[F]): F[A] = F.map(value)(_.fold(identity, ev.apply))
 
   /**
-   * Similar to [[Either.combine]] but mapped over an `F` context.
+   * Similar to `Either#combine` but mapped over an `F` context.
    *
    * Examples:
    * {{{
diff --git a/core/src/main/scala/cats/data/Ior.scala b/core/src/main/scala/cats/data/Ior.scala
index f504f78bfc..88801735e2 100644
--- a/core/src/main/scala/cats/data/Ior.scala
+++ b/core/src/main/scala/cats/data/Ior.scala
@@ -18,7 +18,7 @@ import scala.annotation.tailrec
  *
  * `A [[Ior]] B` is isomorphic to `Either[Either[A, B], (A, B)]`, but provides methods biased toward `B`
  * values, regardless of whether the `B` values appear in a [[Ior.Right Right]] or a [[Ior.Both Both]].
- * The isomorphic [[Either]] form can be accessed via the [[unwrap]] method.
+ * The isomorphic [[scala.util.Either]] form can be accessed via the [[unwrap]] method.
  */
 sealed abstract class Ior[+A, +B] extends Product with Serializable {
 
diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
index 2058904920..71cb64515b 100644
--- a/core/src/main/scala/cats/instances/either.scala
+++ b/core/src/main/scala/cats/instances/either.scala
@@ -81,6 +81,7 @@ trait EitherInstances extends EitherInstances1 {
       override def ensure[B](fab: Either[A, B])(error: => A)(predicate: B => Boolean): Either[A, B] =
         fab.ensure(error)(predicate)
     }
+  // scalastyle:on method.length
 
   implicit def catsStdOrderForEither[A, B](implicit A: Order[A], B: Order[B]): Order[Either[A, B]] = new Order[Either[A, B]] {
     def compare(x: Either[A, B], y: Either[A, B]): Int = x.fold(
diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index 2d44f219b9..acaa79f81b 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -8,57 +8,94 @@ import scala.util.{Failure, Success, Try}
 trait EitherSyntax {
   implicit def catsSyntaxEither[A, B](eab: Either[A, B]): EitherOps[A, B] = new EitherOps(eab)
 
-  implicit def catsSyntaxEitherObject(either: Either.type ): EitherObjectOps = new EitherObjectOps(either)
+  implicit def catsSyntaxEitherObject(either: Either.type): EitherObjectOps = new EitherObjectOps(either) // scalastyle:off ensure.single.space.after.token
 }
 
 final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
-  def foreach(f: B => Unit): Unit = eab.fold(_ => (), f)
+  def foreach(f: B => Unit): Unit = eab match {
+    case Left(_)  => ()
+    case Right(b) => f(b)
+  }
 
-  def getOrElse[BB >: B](default: => BB): BB = eab.fold(_ => default, identity)
+  def getOrElse(default: => B): B = eab match {
+    case Left(_)  => default
+    case Right(b) => b
+  }
 
-  def orElse[C, BB >: B](fallback: => Either[C, BB]): Either[C, BB]= eab match {
-    case Left(_)      => fallback
-    case r @ Right(_) => r.asInstanceOf[Either[C, BB]]
+  def orElse[C](fallback: => Either[C, B]): Either[C, B] = eab match {
+    case Left(_)  => fallback
+    case Right(_) => leftCast[C]
   }
 
-  def recover[BB >: B](pf: PartialFunction[A, BB]): Either[A, BB] = eab match {
+  def recover(pf: PartialFunction[A, B]): Either[A, B] = eab match {
     case Left(a) if pf.isDefinedAt(a) => Right(pf(a))
     case _                            => eab
   }
 
-  def recoverWith[AA >: A, BB >: B](pf: PartialFunction[A, Either[AA, BB]]): Either[AA, BB] = eab match {
+  def recoverWith(pf: PartialFunction[A, Either[A, B]]): Either[A, B] = eab match {
     case Left(a) if pf.isDefinedAt(a) => pf(a)
     case _                            => eab
   }
 
-  def valueOr[BB >: B](f: A => BB): BB = eab.fold(f, identity)
+  def valueOr(f: A => B): B = eab match {
+    case Left(a)  => f(a)
+    case Right(b) => b
+  }
 
-  def forall(f: B => Boolean): Boolean = eab.fold(_ => true, f)
+  def forall(f: B => Boolean): Boolean = eab match {
+    case Left(_)  => true
+    case Right(b) => f(b)
+  }
 
-  def exists(f: B => Boolean): Boolean = eab.fold(_ => false, f)
+  def exists(f: B => Boolean): Boolean = eab match {
+    case Left(_)  => false
+    case Right(b) => f(b)
+  }
 
-  def ensure[AA >: A](onFailure: => AA)(f: B => Boolean): Either[AA, B] =
-    eab.fold(_ => eab, b => if (f(b)) eab else Left(onFailure))
+  def ensure(onFailure: => A)(f: B => Boolean): Either[A, B] = eab match {
+    case Left(_)  => eab
+    case Right(b) => if (f(b)) eab else Left(onFailure)
+  }
 
-  def toIor: A Ior B = eab.fold(Ior.left, Ior.right)
+  def toIor: A Ior B = eab match {
+    case Left(a)  => Ior.left(a)
+    case Right(b) => Ior.right(b)
+  }
 
-  def toOption: Option[B] = eab.fold(_ => None, Some(_))
+  def toOption: Option[B] = eab match {
+    case Left(_)  => None
+    case Right(b) => Some(b)
+  }
 
-  def toList: List[B] = eab.fold(_ => Nil, _ :: Nil)
+  def toList: List[B] = eab match {
+    case Left(_)  => Nil
+    case Right(b) => List(b)
+  }
 
-  def toTry(implicit ev: A <:< Throwable): Try[B] = eab.fold(a => Failure(ev(a)), Success(_))
+  def toTry(implicit ev: A <:< Throwable): Try[B] = eab match {
+    case Left(a)  => Failure(ev(a))
+    case Right(b) => Success(b)
+  }
 
-  def toValidated: Validated[A, B] = eab.fold(Validated.Invalid.apply, Validated.Valid.apply)
+  def toValidated: Validated[A, B] = eab match {
+    case Left(a)  => Validated.invalid(a)
+    case Right(b) => Validated.valid(b)
+  }
 
-  /** Returns a [[ValidatedNel]] representation of this disjunction with the `Left` value
-   * as a single element on the `Invalid` side of the [[NonEmptyList]]. */
-  def toValidatedNel[AA >: A]: ValidatedNel[AA, B] = eab.fold(Validated.invalidNel, Validated.valid)
+  /** Returns a [[cats.data.ValidatedNel]] representation of this disjunction with the `Left` value
+   * as a single element on the `Invalid` side of the [[cats.data.NonEmptyList]]. */
+  def toValidatedNel: ValidatedNel[A, B] = eab match {
+    case Left(a)  => Validated.invalidNel(a)
+    case Right(b) => Validated.valid(b)
+  }
 
   def withValidated[AA, BB](f: Validated[A, B] => Validated[AA, BB]): Either[AA, BB] =
     f(toValidated).toEither
 
-  def to[F[_], BB >: B](implicit F: Alternative[F]): F[BB] =
-    eab.fold(_ => F.empty, F.pure)
+  def to[F[_]](implicit F: Alternative[F]): F[B] = eab match {
+    case Left(_)  => F.empty
+    case Right(b) => F.pure(b)
+  }
 
   def bimap[C, D](fa: A => C, fb: B => D): Either[C, D] = eab match {
     case Left(a)  => Left(fa(a))
@@ -66,51 +103,79 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def map[C](f: B => C): Either[A, C] = eab match {
-    case l @ Left(_) => l.asInstanceOf[Either[A, C]]
-    case Right(b)    => Right(f(b))
+    case Left(_)  => rightCast[C]
+    case Right(b) => Right(f(b))
   }
 
-  def map2Eval[AA >: A, C, Z](fc: Eval[Either[AA, C]])(f: (B, C) => Z): Eval[Either[AA, Z]] =
+  def map2Eval[C, Z](fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
     eab match {
-      case l @ Left(_) => Now(l.asInstanceOf[Either[AA, Z]])
-      case Right(b)    => fc.map(either => new EitherOps(either).map(f(b, _)))
+      case Left(_)  => Now(rightCast[Z])
+      case Right(b) => fc.map(either => new EitherOps(either).map(f(b, _)))
     }
 
   def leftMap[C](f: A => C): Either[C, B] = eab match {
-    case Left(a)      => Left(f(a))
-    case r @ Right(_) => r.asInstanceOf[Either[C, B]]
+    case Left(a)  => Left(f(a))
+    case Right(_) => leftCast[C]
   }
 
-  def flatMap[AA >: A, D](f: B => Either[AA, D]): Either[AA, D] = eab match {
-    case l @ Left(_) => l.asInstanceOf[Either[AA, D]]
-    case Right(b)    => f(b)
+  def flatMap[D](f: B => Either[A, D]): Either[A, D] = eab match {
+    case Left(_)  => rightCast[D]
+    case Right(b) => f(b)
   }
 
-  def compare[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: Order[AA], BB: Order[BB]): Int = eab.fold(
-    a => that.fold(AA.compare(a, _), _ => -1),
-    b => that.fold(_ => 1, BB.compare(b, _))
-  )
+  def compare(that: Either[A, B])(implicit A: Order[A], B: Order[B]): Int = eab match {
+    case Left(a1)  =>
+      that match {
+        case Left(a2) => A.compare(a1, a2)
+        case Right(_) => -1
+      }
+    case Right(b1) =>
+      that match {
+        case Left(_)   => 1
+        case Right(b2) => B.compare(b1, b2)
+      }
+  }
 
-  def partialCompare[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: PartialOrder[AA], BB: PartialOrder[BB]): Double =
-    eab.fold(
-      a => that.fold(AA.partialCompare(a, _), _ => -1),
-      b => that.fold(_ => 1, BB.partialCompare(b, _))
-    )
+  def partialCompare(that: Either[A, B])(implicit A: PartialOrder[A], B: PartialOrder[B]): Double = eab match {
+    case Left(a1)  =>
+      that match {
+        case Left(a2) => A.partialCompare(a1, a2)
+        case Right(_) => -1
+      }
+    case Right(b1) =>
+      that match {
+        case Left(_)   => 1
+        case Right(b2) => B.partialCompare(b1, b2)
+      }
+  }
 
-  def ===[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: Eq[AA], BB: Eq[BB]): Boolean = eab.fold(
-    a => that.fold(AA.eqv(a, _), _ => false),
-    b => that.fold(_ => false, BB.eqv(b, _))
-  )
+  def ===(that: Either[A, B])(implicit A: Eq[A], B: Eq[B]): Boolean = eab match {
+    case Left(a1)  =>
+      that match {
+        case Left(a2) => A.eqv(a1, a2)
+        case Right(_) => false
+      }
+    case Right(b1) =>
+      that match {
+        case Left(_)   => false
+        case Right(b2) => B.eqv(b1, b2)
+      }
+  }
 
-  def traverse[F[_], AA >: A, D](f: B => F[D])(implicit F: Applicative[F]): F[Either[AA, D]] = eab match {
-    case l @ Left(_) => F.pure(l.asInstanceOf[Either[AA, D]])
+  def traverse[F[_], D](f: B => F[D])(implicit F: Applicative[F]): F[Either[A, D]] = eab match {
+    case Left(_)  => F.pure(rightCast[D])
     case Right(b) => F.map(f(b))(Right(_))
   }
 
-  def foldLeft[C](c: C)(f: (C, B) => C): C = eab.fold(_ => c, f(c, _))
+  def foldLeft[C](c: C)(f: (C, B) => C): C = eab match {
+    case Left(_)  => c
+    case Right(b) => f(c, b)
+  }
 
-  def foldRight[C](lc: Eval[C])(f: (B, Eval[C]) => Eval[C]): Eval[C] =
-    eab.fold(_ => lc, b => f(b, lc))
+  def foldRight[C](lc: Eval[C])(f: (B, Eval[C]) => Eval[C]): Eval[C] = eab match {
+    case Left(_)  => lc
+    case Right(b) => f(b, lc)
+  }
 
   /**
    * Combine with another `Either` value.
@@ -144,20 +209,20 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
    * res3: Either[String, Int] = Right(7)
    * }}}
    */
-  final def combine[AA >: A, BB >: B](that: Either[AA, BB])(implicit BB: Semigroup[BB]): Either[AA, BB] = eab match {
+  final def combine(that: Either[A, B])(implicit B: Semigroup[B]): Either[A, B] = eab match {
     case left @ Left(_) => left
     case Right(b1) => that match {
       case left @ Left(_) => left
-      case Right(b2) => Right(BB.combine(b1, b2))
+      case Right(b2) => Right(B.combine(b1, b2))
     }
   }
 
-  def show[AA >: A, BB >: B](implicit AA: Show[AA], BB: Show[BB]): String = eab.fold(
-    a => s"Left(${AA.show(a)})",
-    b => s"Right(${BB.show(b)})"
-  )
+  def show(implicit A: Show[A], B: Show[B]): String = eab match {
+    case Left(a)  => s"Left(${A.show(a)})"
+    case Right(b) => s"Right(${B.show(b)})"
+  }
 
-  def ap[AA >: A, BB >: B, C](that: Either[AA, BB => C]): Either[AA, C] = (new EitherOps(that)).flatMap(this.map)
+  def ap[C](that: Either[A, B => C]): Either[A, C] = (new EitherOps(that)).flatMap(this.map)
 
   /**
    * Convert a `scala.util.Either` into a [[cats.data.Xor]].
@@ -179,7 +244,7 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   def toXor: A Xor B = Xor.fromEither(eab)
 
   /**
-   * Transform the `Either` into a [[EitherT]] while lifting it into the specified Applicative.
+   * Transform the `Either` into a [[cats.data.EitherT]] while lifting it into the specified Applicative.
    *
    * {{{
    * scala> import cats.implicits._
@@ -189,9 +254,13 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
    * }}}
    */
   def toEitherT[F[_]: Applicative]: EitherT[F, A, B] = EitherT.fromEither(eab)
+
+  private[cats] def leftCast[C]: Either[C, B] = eab.asInstanceOf[Either[C, B]]
+
+  private[cats] def rightCast[C]: Either[A, C] = eab.asInstanceOf[Either[A, C]]
 }
 
-final class EitherObjectOps(val either: Either.type ) extends AnyVal {
+final class EitherObjectOps(val either: Either.type) extends AnyVal { // scalastyle:off ensure.single.space.after.token
   def left[A, B](a: A): Either[A, B] = Left(a)
 
   def right[A, B](b: B): Either[A, B] = Right(b)
@@ -230,8 +299,10 @@ final class EitherObjectOps(val either: Either.type ) extends AnyVal {
    * Converts an `Option[B]` to an `A Xor B`, where the provided `ifNone` values is returned on
    * the left of the `Xor` when the specified `Option` is `None`.
    */
-  def fromOption[A, B](o: Option[B], ifNone: => A): Either[A, B] =
-    o.fold(left[A, B](ifNone))(right)
+  def fromOption[A, B](o: Option[B], ifNone: => A): Either[A, B] = o match {
+    case None    => left[A, B](ifNone)
+    case Some(a) => right(a)
+  }
 }
 
 final class CatchOnlyPartiallyApplied[T] private[syntax] {
diff --git a/tests/src/test/scala/cats/tests/EitherTests.scala b/tests/src/test/scala/cats/tests/EitherTests.scala
index 8e4641e911..3f91e21f1f 100644
--- a/tests/src/test/scala/cats/tests/EitherTests.scala
+++ b/tests/src/test/scala/cats/tests/EitherTests.scala
@@ -227,13 +227,13 @@ class EitherTests extends CatsSuite {
 
   test("to consistent with toList") {
     forAll { (x: Either[Int, String]) =>
-      x.to[List, String] should === (x.toList)
+      x.to[List] should === (x.toList)
     }
   }
 
   test("to consistent with toOption") {
     forAll { (x: Either[Int, String]) =>
-      x.to[Option, String] should === (x.toOption)
+      x.to[Option] should === (x.toOption)
     }
   }
 }

From d4df094775642d569385461fd6f0008aefe27f5a Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Tue, 16 Aug 2016 11:47:44 -0700
Subject: [PATCH 5/7] Eval uses defaultTailRecM

---
 core/src/main/scala/cats/Eval.scala | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/core/src/main/scala/cats/Eval.scala b/core/src/main/scala/cats/Eval.scala
index 21243fd943..36348160fe 100644
--- a/core/src/main/scala/cats/Eval.scala
+++ b/core/src/main/scala/cats/Eval.scala
@@ -301,11 +301,7 @@ private[cats] trait EvalInstances extends EvalInstances0 {
       def flatMap[A, B](fa: Eval[A])(f: A => Eval[B]): Eval[B] = fa.flatMap(f)
       def extract[A](la: Eval[A]): A = la.value
       def coflatMap[A, B](fa: Eval[A])(f: Eval[A] => B): Eval[B] = Later(f(fa))
-      def tailRecM[A, B](a: A)(f: A => Eval[Either[A, B]]): Eval[B] =
-        f(a).flatMap(_ match {
-          case Left(a1) => tailRecM(a1)(f) // recursion OK here, since flatMap is lazy
-          case Right(b) => Eval.now(b)
-        })
+      def tailRecM[A, B](a: A)(f: A => Eval[Either[A, B]]): Eval[B] = defaultTailRecM(a)(f)
     }
 
   implicit def catsOrderForEval[A: Order]: Order[Eval[A]] =

From eedd057830717f87ccf24149a9bc1e15816e2579 Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Tue, 16 Aug 2016 21:32:04 -0700
Subject: [PATCH 6/7] Safer Either casting

---
 core/src/main/scala/cats/data/EitherT.scala   |  3 +-
 .../main/scala/cats/instances/either.scala    |  6 ++--
 core/src/main/scala/cats/syntax/either.scala  | 36 ++++++++++---------
 3 files changed, 24 insertions(+), 21 deletions(-)

diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
index c9649e926f..2bdc04655e 100644
--- a/core/src/main/scala/cats/data/EitherT.scala
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -3,6 +3,7 @@ package data
 
 import cats.functor.Bifunctor
 import cats.instances.either._
+import cats.syntax.EitherUtil
 import cats.syntax.either._
 
 /**
@@ -72,7 +73,7 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
 
   def flatMap[D](f: B => EitherT[F, A, D])(implicit F: Monad[F]): EitherT[F, A, D] =
     EitherT(F.flatMap(value) {
-      case l @ Left(_) => F.pure((l: Either[A, B]).rightCast[D])
+      case l @ Left(_) => F.pure(EitherUtil.leftCast(l))
       case Right(b) => f(b).value
     })
 
diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
index 71cb64515b..5acaa3795d 100644
--- a/core/src/main/scala/cats/instances/either.scala
+++ b/core/src/main/scala/cats/instances/either.scala
@@ -1,6 +1,7 @@
 package cats
 package instances
 
+import cats.syntax.EitherUtil
 import cats.syntax.either._
 import scala.annotation.tailrec
 
@@ -54,10 +55,7 @@ trait EitherInstances extends EitherInstances1 {
 
       override def map2Eval[B, C, Z](fb: Either[A, B], fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
         fb match {
-          // This should be safe, but we are forced to use `asInstanceOf`,
-          // because `Left[+A, +B]` extends Either[A, B] instead of
-          // `Either[A, Nothing]`
-          case l @ Left(_) => Now(l.asInstanceOf[Either[A, Z]])
+          case l @ Left(_) => Now(EitherUtil.leftCast(l))
           case Right(b) => fc.map(_.right.map(f(b, _)))
         }
 
diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index acaa79f81b..cbac3404b8 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -23,8 +23,8 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def orElse[C](fallback: => Either[C, B]): Either[C, B] = eab match {
-    case Left(_)  => fallback
-    case Right(_) => leftCast[C]
+    case Left(_)      => fallback
+    case r @ Right(_) => EitherUtil.rightCast(r)
   }
 
   def recover(pf: PartialFunction[A, B]): Either[A, B] = eab match {
@@ -103,24 +103,24 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def map[C](f: B => C): Either[A, C] = eab match {
-    case Left(_)  => rightCast[C]
-    case Right(b) => Right(f(b))
+    case l @ Left(_) => EitherUtil.leftCast(l)
+    case Right(b)    => Right(f(b))
   }
 
   def map2Eval[C, Z](fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
     eab match {
-      case Left(_)  => Now(rightCast[Z])
-      case Right(b) => fc.map(either => new EitherOps(either).map(f(b, _)))
+      case l @ Left(_) => Now(EitherUtil.leftCast(l))
+      case Right(b)    => fc.map(either => new EitherOps(either).map(f(b, _)))
     }
 
   def leftMap[C](f: A => C): Either[C, B] = eab match {
-    case Left(a)  => Left(f(a))
-    case Right(_) => leftCast[C]
+    case Left(a)      => Left(f(a))
+    case r @ Right(_) => EitherUtil.rightCast(r)
   }
 
   def flatMap[D](f: B => Either[A, D]): Either[A, D] = eab match {
-    case Left(_)  => rightCast[D]
-    case Right(b) => f(b)
+    case l @ Left(_) => EitherUtil.leftCast(l)
+    case Right(b)    => f(b)
   }
 
   def compare(that: Either[A, B])(implicit A: Order[A], B: Order[B]): Int = eab match {
@@ -163,8 +163,8 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def traverse[F[_], D](f: B => F[D])(implicit F: Applicative[F]): F[Either[A, D]] = eab match {
-    case Left(_)  => F.pure(rightCast[D])
-    case Right(b) => F.map(f(b))(Right(_))
+    case l @ Left(_) => F.pure(EitherUtil.leftCast(l))
+    case Right(b)    => F.map(f(b))(Right(_))
   }
 
   def foldLeft[C](c: C)(f: (C, B) => C): C = eab match {
@@ -254,10 +254,6 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
    * }}}
    */
   def toEitherT[F[_]: Applicative]: EitherT[F, A, B] = EitherT.fromEither(eab)
-
-  private[cats] def leftCast[C]: Either[C, B] = eab.asInstanceOf[Either[C, B]]
-
-  private[cats] def rightCast[C]: Either[A, C] = eab.asInstanceOf[Either[A, C]]
 }
 
 final class EitherObjectOps(val either: Either.type) extends AnyVal { // scalastyle:off ensure.single.space.after.token
@@ -314,3 +310,11 @@ final class CatchOnlyPartiallyApplied[T] private[syntax] {
         Left(t.asInstanceOf[T])
     }
 }
+
+private[cats] object EitherUtil {
+  /** Cast the *right* type parameter of a `Left`. */
+  def leftCast[A, B, C](l: Left[A, B]): Either[A, C] = l.asInstanceOf[Left[A, C]]
+
+  /** Cast the *left* type parameter of a `Right` */
+  def rightCast[A, B, C](r: Right[A, B]): Either[C, B] = r.asInstanceOf[Right[C, B]]
+}

From ba1a7860a34829755fd5adc9d0397c12adb8b2eb Mon Sep 17 00:00:00 2001
From: Adelbert Chang <adelbertc@gmail.com>
Date: Tue, 16 Aug 2016 22:02:26 -0700
Subject: [PATCH 7/7] Expose Left/Right cast syntax - casting naming indicates
 which type parameter to cast

---
 core/src/main/scala/cats/data/EitherT.scala   |  3 +-
 .../main/scala/cats/instances/either.scala    |  2 +-
 core/src/main/scala/cats/syntax/either.scala  | 33 +++++++++++++------
 .../test/scala/cats/tests/EitherTests.scala   | 12 +++++++
 4 files changed, 37 insertions(+), 13 deletions(-)

diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
index 2bdc04655e..66915a7404 100644
--- a/core/src/main/scala/cats/data/EitherT.scala
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -3,7 +3,6 @@ package data
 
 import cats.functor.Bifunctor
 import cats.instances.either._
-import cats.syntax.EitherUtil
 import cats.syntax.either._
 
 /**
@@ -73,7 +72,7 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
 
   def flatMap[D](f: B => EitherT[F, A, D])(implicit F: Monad[F]): EitherT[F, A, D] =
     EitherT(F.flatMap(value) {
-      case l @ Left(_) => F.pure(EitherUtil.leftCast(l))
+      case l @ Left(_) => F.pure(l.rightCast)
       case Right(b) => f(b).value
     })
 
diff --git a/core/src/main/scala/cats/instances/either.scala b/core/src/main/scala/cats/instances/either.scala
index 5acaa3795d..d5b3cd628f 100644
--- a/core/src/main/scala/cats/instances/either.scala
+++ b/core/src/main/scala/cats/instances/either.scala
@@ -55,7 +55,7 @@ trait EitherInstances extends EitherInstances1 {
 
       override def map2Eval[B, C, Z](fb: Either[A, B], fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
         fb match {
-          case l @ Left(_) => Now(EitherUtil.leftCast(l))
+          case l @ Left(_) => Now(EitherUtil.rightCast(l))
           case Right(b) => fc.map(_.right.map(f(b, _)))
         }
 
diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index cbac3404b8..f4809f410a 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -9,6 +9,10 @@ trait EitherSyntax {
   implicit def catsSyntaxEither[A, B](eab: Either[A, B]): EitherOps[A, B] = new EitherOps(eab)
 
   implicit def catsSyntaxEitherObject(either: Either.type): EitherObjectOps = new EitherObjectOps(either) // scalastyle:off ensure.single.space.after.token
+
+  implicit def catsSyntaxLeft[A, B](left: Left[A, B]): LeftOps[A, B] = new LeftOps(left)
+
+  implicit def catsSyntaxRight[A, B](right: Right[A, B]): RightOps[A, B] = new RightOps(right)
 }
 
 final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
@@ -24,7 +28,7 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
 
   def orElse[C](fallback: => Either[C, B]): Either[C, B] = eab match {
     case Left(_)      => fallback
-    case r @ Right(_) => EitherUtil.rightCast(r)
+    case r @ Right(_) => EitherUtil.leftCast(r)
   }
 
   def recover(pf: PartialFunction[A, B]): Either[A, B] = eab match {
@@ -103,23 +107,23 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def map[C](f: B => C): Either[A, C] = eab match {
-    case l @ Left(_) => EitherUtil.leftCast(l)
+    case l @ Left(_) => EitherUtil.rightCast(l)
     case Right(b)    => Right(f(b))
   }
 
   def map2Eval[C, Z](fc: Eval[Either[A, C]])(f: (B, C) => Z): Eval[Either[A, Z]] =
     eab match {
-      case l @ Left(_) => Now(EitherUtil.leftCast(l))
+      case l @ Left(_) => Now(EitherUtil.rightCast(l))
       case Right(b)    => fc.map(either => new EitherOps(either).map(f(b, _)))
     }
 
   def leftMap[C](f: A => C): Either[C, B] = eab match {
     case Left(a)      => Left(f(a))
-    case r @ Right(_) => EitherUtil.rightCast(r)
+    case r @ Right(_) => EitherUtil.leftCast(r)
   }
 
   def flatMap[D](f: B => Either[A, D]): Either[A, D] = eab match {
-    case l @ Left(_) => EitherUtil.leftCast(l)
+    case l @ Left(_) => EitherUtil.rightCast(l)
     case Right(b)    => f(b)
   }
 
@@ -163,7 +167,7 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
   }
 
   def traverse[F[_], D](f: B => F[D])(implicit F: Applicative[F]): F[Either[A, D]] = eab match {
-    case l @ Left(_) => F.pure(EitherUtil.leftCast(l))
+    case l @ Left(_) => F.pure(EitherUtil.rightCast(l))
     case Right(b)    => F.map(f(b))(Right(_))
   }
 
@@ -311,10 +315,19 @@ final class CatchOnlyPartiallyApplied[T] private[syntax] {
     }
 }
 
+final class LeftOps[A, B](val left: Left[A, B]) extends AnyVal {
+  /** Cast the right type parameter of the `Left`. */
+  def rightCast[C]: Either[A, C] = left.asInstanceOf[Either[A, C]]
+}
+
+final class RightOps[A, B](val right: Right[A, B]) extends AnyVal {
+  /** Cast the left type parameter of the `Right`. */
+  def leftCast[C]: Either[C, B] = right.asInstanceOf[Either[C, B]]
+}
+
+/** Convenience methods to use `Either` syntax inside `Either` syntax definitions. */
 private[cats] object EitherUtil {
-  /** Cast the *right* type parameter of a `Left`. */
-  def leftCast[A, B, C](l: Left[A, B]): Either[A, C] = l.asInstanceOf[Left[A, C]]
+  def leftCast[A, B, C](r: Right[A, B]): Either[C, B] = new RightOps(r).leftCast[C]
 
-  /** Cast the *left* type parameter of a `Right` */
-  def rightCast[A, B, C](r: Right[A, B]): Either[C, B] = r.asInstanceOf[Right[C, B]]
+  def rightCast[A, B, C](l: Left[A, B]): Either[A, C] = new LeftOps(l).rightCast[C]
 }
diff --git a/tests/src/test/scala/cats/tests/EitherTests.scala b/tests/src/test/scala/cats/tests/EitherTests.scala
index 3f91e21f1f..5db908b543 100644
--- a/tests/src/test/scala/cats/tests/EitherTests.scala
+++ b/tests/src/test/scala/cats/tests/EitherTests.scala
@@ -44,6 +44,18 @@ class EitherTests extends CatsSuite {
   checkAll("Either[Int, String]", orderLaws.partialOrder(partialOrder))
   checkAll("Either[Int, String]", orderLaws.order(order))
 
+  test("Left/Right cast syntax") {
+    forAll { (e: Either[Int, String]) =>
+      e match {
+        case l @ Left(_)  =>
+          l.rightCast[Double]: Either[Int, Double]
+          assert(true)
+        case r @ Right(_) =>
+          r.leftCast[List[Byte]]: Either[List[Byte], String]
+          assert(true)
+      }
+    }
+  }
 
   test("implicit instances resolve specifically") {
     val eq = catsStdEqForEither[Int, String]